Right click or shift right click with the stamp causes a crash when Chisel and CTM are installed.
Does not crash with only CTM, crashes with Chisel. Chisel is dependent on CTM.

Launching with MultiMC if it matters

Minecraft 1.12.2
Forge 14.23.5.2854

MCPaint 1.2.4
Chisel 1.0.2.45
CTM 1.0.2.31

Crashlog: crash-2021-02-23_20.45.18-client.txt

Reproducing:
Install the above and run.
Use the brush to paint anything
Right click or shift right click with the stamp