This happens with clearAllButtons()
The crash happens when entering options, multiplayer, single player etc screens and then trying to go back to the title screen.
I also tried re-adding a button with onClicked = realms() but that did not prevent the crash.

Only slight-gui-modifications-2.6.1+ dependencies + ModMenu and FabricAPI

Description: mouseClicked event handler
java.lang.NullPointerException: Cannot invoke "net.minecraft.client.MinecraftClient.getRealmsPeriodicCheckers()" because "this.client" is null
at net.minecraft.client.realms.gui.screen.RealmsNotificationsScreen.onDisplayed(RealmsNotificationsScreen.java:54)
https://hastebin.com/share/irobufesum.yaml