java.lang.NullPointerException: mouseClicked event handler

What the log looks like

java\.lang\.NullPointerException: mouseClicked event handler
Description: mouseClicked event handler

How to fix it

1. Open the crash report and read the very first "at ..." line directly below "java.lang.NullPointerException: mouseClicked event handler" to identify which mod or vanilla subsystem actually crashed.
2. If that first line starts with a mod package (such as mezz.jei or me.shedaniel.rei), update that mod to the newest build for your exact Minecraft version, or temporarily move its .jar out of the mods folder to confirm it is the cause.
3. If the crash occurred when clicking Done on the Resource Packs screen, open options.txt in your Minecraft folder, set resourcePacks:[] to disable the broken pack on startup, and update any recipe or UI mods before re-enabling the pack.
4. If the first line points to net.minecraft.world.storage (such as WorldInfo or SaveHandler) when clicking Play or Optimize World, back up your saves folder and replace level.dat with a copy of level.dat_old, or upgrade very old worlds (like 1.12.2) one major version at a time.
5. If the top line points to net.minecraftforge.fml.network when joining a world, update Forge or NeoForge to the latest recommended build for your Minecraft version and test without OptiFine installed.

Check your own log