workaround [#25845] Empty UI panels

- now writing to RNA is disabled when inside render() call.
- disallow calling operators when writes are disabled.

Rendering runs in a thread so running operators from the thread is not safe unless rendering becomes a blocking operator again.

source/blender/python/intern/bpy_operator.c

@@ -138,6 +138,11 @@ static PyObject *pyop_call(PyObject *UNUSED(self), PyObject *args)
 		return NULL;
 	}
 	
+	if(!pyrna_write_check()) {
+		PyErr_Format(PyExc_SystemError, "Calling operator \"bpy.ops.%s\" error, can't modify blend data in this state (drawing/rendering)", opname);
+		return NULL;
+	}
+
 	if(context_str) {
 		if(RNA_enum_value_from_id(operator_context_items, context_str, &context)==0) {
 			char *enum_str= BPy_enum_as_string(operator_context_items);