-wip on addon editor

-fixes instantiable subclasses not working, as reported in #3871

modules/gdscript/gd_compiler.cpp

@@ -1712,6 +1712,8 @@ Error GDCompiler::_parse_class(GDScript *p_script,GDScript *p_owner,const GDPars
 		}
 	}
 #endif
+
+	p_script->valid=true;
 	return OK;
 }