I set the file association of a .pxc file to open in Pixel Composer, but when I try it crashes with this error:

___________________________________________ 
############################################################################################ 
ERROR in 
action number 1 
of Alarm Event for alarm 2 
for object o_main:  
trying to index variable that is not an array############################################################################################ 
gml_Script_load_file_path_gml_Object_o_main_Create_0 (line 93) 
gml_Object_o_main_Alarm_2 (line 2)

Is this an actual error or does it just not support "Open as..."?

That’s a error, you should be able to use open as to open file in PXC, I’ll look in to it.