No other mods installed,
with error
"The game crashed whilst initializing game
Error: java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
Exit Code: -1"

Thank you for the bug report and sorry for the delayed response. Please could you try updating forge to the latest version for the Minecraft version you are using. That should fix the issue you are seeing here :)

Time: 01/12/20 22:55
Description: Initializing game

java.lang.RuntimeException: java.lang.reflect.InvocationTargetException
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:78) ~[?:31.1] {re:classloading}
at net.minecraftforge.fml.ModLoader.buildModContainerFromTOML(ModLoader.java:234) ~[?:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.lambda$buildMods$26(ModLoader.java:214) ~[?:?] {re:classloading}
at net.minecraftforge.fml.ModLoader$$Lambda$2152/1873631950.apply(Unknown Source) ~[?:?] {}
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[?:1.8.0_51] {}
at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1683) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502) ~[?:1.8.0_51] {}
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_51] {}
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.ModLoader.buildMods(ModLoader.java:216) ~[?:?] {re:classloading}
at net.minecraftforge.fml.ModLoader.lambda$gatherAndInitializeMods$18(ModLoader.java:173) ~[?:?] {re:classloading}
at net.minecraftforge.fml.ModLoader$$Lambda$2149/778350106.apply(Unknown Source) ~[?:?] {}
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[?:1.8.0_51] {}
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193) ~[?:1.8.0_51] {}
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502) ~[?:1.8.0_51] {}
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708) ~[?:1.8.0_51] {}
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[?:1.8.0_51] {}
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.ModLoader.gatherAndInitializeMods(ModLoader.java:175) ~[?:?] {re:classloading}
at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$2(ClientModLoader.java:97) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraftforge.fml.client.ClientModLoader$$Lambda$2056/251266229.run(Unknown Source) ~[?:?] {}
at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$5(ClientModLoader.java:113) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraftforge.fml.client.ClientModLoader$$Lambda$2057/176203926.run(Unknown Source) ~[?:?] {}
at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:97) ~[?:?] {re:classloading,pl:runtimedistcleaner:A}
at net.minecraft.client.Minecraft.(Minecraft.java:393) [?:?] {re:classloading,pl:accesstransformer:B,pl:runtimedistcleaner:A}
at net.minecraft.client.main.Main.main(SourceFile:166) [?:?] {re:classloading}
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[?:1.8.0_51] {}
at java.lang.reflect.Method.invoke(Method.java:497) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:56) [forge-1.15.2-31.1.0.jar:31.1] {}
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$395/1925785585.call(Unknown Source) [forge-1.15.2-31.1.0.jar:31.1] {}
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37) [modlauncher-5.0.0-milestone.4.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54) [modlauncher-5.0.0-milestone.4.jar:?] {}
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72) [modlauncher-5.0.0-milestone.4.jar:?] {}
at cpw.mods.modlauncher.Launcher.run(Launcher.java:81) [modlauncher-5.0.0-milestone.4.jar:?] {}
at cpw.mods.modlauncher.Launcher.main(Launcher.java:65) [modlauncher-5.0.0-milestone.4.jar:?] {}
Caused by: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:1.8.0_51] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:73) ~[?:31.1] {re:classloading}
... 40 more
Caused by: net.minecraftforge.fml.ModLoadingException: Double Slabs has class loading errors
ยง7net/minecraftforge/fml/DistExecutor$SafeSupplier
at net.minecraftforge.fml.javafmlmod.FMLModContainer.(FMLModContainer.java:77) ~[?:31.1] {re:classloading}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:1.8.0_51] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:73) ~[?:31.1] {re:classloading}
... 40 more
Caused by: java.lang.NoClassDefFoundError: net/minecraftforge/fml/DistExecutor$SafeSupplier
at cjminecraft.doubleslabs.common.DoubleSlabs.(DoubleSlabs.java:33) ~[?:2.11.7] {re:classloading}
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51] {}
at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.(FMLModContainer.java:71) ~[?:31.1] {re:classloading}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:1.8.0_51] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:73) ~[?:31.1] {re:classloading}
... 40 more
Caused by: java.lang.ClassNotFoundException: net.minecraftforge.fml.DistExecutor$SafeSupplier
at java.lang.ClassLoader.findClass(ClassLoader.java:530) ~[?:1.8.0_51] {}
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_51] {}
at cpw.mods.modlauncher.TransformingClassLoader.loadClass(TransformingClassLoader.java:101) ~[modlauncher-5.0.0-milestone.4.jar:?] {}
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_51] {}
at cjminecraft.doubleslabs.common.DoubleSlabs.(DoubleSlabs.java:33) ~[?:2.11.7] {re:classloading}
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_51] {}
at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLModContainer.(FMLModContainer.java:71) ~[?:31.1] {re:classloading}
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_51] {}
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[?:1.8.0_51] {}
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[?:1.8.0_51] {}
at java.lang.reflect.Constructor.newInstance(Constructor.java:422) ~[?:1.8.0_51] {}
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:73) ~[?:31.1] {re:classloading}
... 40 more

A detailed walkthrough of the error, its code path and all known details is as follows:

-- Head --
Thread: Render thread
Stacktrace:
at net.minecraftforge.fml.javafmlmod.FMLJavaModLanguageProvider$FMLModTarget.loadMod(FMLJavaModLanguageProvider.java:78)
at net.minecraftforge.fml.ModLoader.buildModContainerFromTOML(ModLoader.java:234)
at net.minecraftforge.fml.ModLoader.lambda$buildMods$26(ModLoader.java:214)
at net.minecraftforge.fml.ModLoader$$Lambda$2152/1873631950.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1683)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at net.minecraftforge.fml.ModLoader.buildMods(ModLoader.java:216)
at net.minecraftforge.fml.ModLoader.lambda$gatherAndInitializeMods$18(ModLoader.java:173)
at net.minecraftforge.fml.ModLoader$$Lambda$2149/778350106.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at net.minecraftforge.fml.ModLoader.gatherAndInitializeMods(ModLoader.java:175)
at net.minecraftforge.fml.client.ClientModLoader.lambda$begin$2(ClientModLoader.java:97)
at net.minecraftforge.fml.client.ClientModLoader$$Lambda$2056/251266229.run(Unknown Source)
at net.minecraftforge.fml.client.ClientModLoader.lambda$createRunnableWithCatch$5(ClientModLoader.java:113)
at net.minecraftforge.fml.client.ClientModLoader$$Lambda$2057/176203926.run(Unknown Source)
at net.minecraftforge.fml.client.ClientModLoader.begin(ClientModLoader.java:97)
at net.minecraft.client.Minecraft.(Minecraft.java:393)

-- Initialization --
Details:
Stacktrace:
at net.minecraft.client.main.Main.main(SourceFile:166)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:497)
at net.minecraftforge.fml.loading.FMLClientLaunchProvider.lambda$launchService$0(FMLClientLaunchProvider.java:56)
at net.minecraftforge.fml.loading.FMLClientLaunchProvider$$Lambda$395/1925785585.call(Unknown Source)
at cpw.mods.modlauncher.LaunchServiceHandlerDecorator.launch(LaunchServiceHandlerDecorator.java:37)
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:54)
at cpw.mods.modlauncher.LaunchServiceHandler.launch(LaunchServiceHandler.java:72)
at cpw.mods.modlauncher.Launcher.run(Launcher.java:81)
at cpw.mods.modlauncher.Launcher.main(Launcher.java:65)