Replicated. This happens when skull lvl 2 has bone and there are no spots left in row.
I looked at the code and there is a bug in gml_Script_EffectTypesAfterDeath. The issue is that when the skull has a bone, it first creates a boner directly under the lvl 2 skull that died. This succeeds. However, after this happens, it will attempt to create a lvl 1 skull. This fails since there are no spots left. (the return value of gml_Script_Summon is invalid). However, the code then tries to apply the effect of the skull - namely to give the lvl 1 skull the bone item. To do this, it attempts to access the skull instance with the return value of gml_Script_Summon. This return value was invalid though since there were no spots left. Hence, it will crash.
Problem snippet:
This same error should effect lvl3 skull also.