I installed the last preview OptiFine 1.15.2 HD U G1 pre14 and when I joined my game Minecraft crash, I read the error report and I noticed that double slabs was the problem I l removed from the mods folder and now I can properly join my world.
crash-2020-05-03_03.40.49-client.txt

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Hi! Thanks for the report. Please try if updating Optifine to the latest version will fix this issue (at the time of writing this that is pre16) and use with the recommended forge version for Optifine which is 31.1.63. I believe in older versions of Optifine there was compatibility issues with forge so please let me know if this issue remains in the latest version! Thanks