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);

source/blender/python/intern/bpy_rna.c

@@ -83,7 +83,19 @@ static int rna_id_write_error(PointerRNA *ptr, PyObject *key)
 	}
 	return FALSE;
 }
+#endif // USE_PEDANTIC_WRITE
 
+
+#ifdef USE_PEDANTIC_WRITE
+int pyrna_write_check(void)
+{
+	return !rna_disallow_writes;
+}
+#else // USE_PEDANTIC_WRITE
+int pyrna_write_check(void)
+{
+	return TRUE;
+}
 #endif // USE_PEDANTIC_WRITE
 
 static Py_ssize_t pyrna_prop_collection_length(BPy_PropertyRNA *self);
@@ -5324,8 +5336,9 @@ static int bpy_class_call(bContext *C, PointerRNA *ptr, FunctionRNA *func, Param
 	PyGILState_STATE gilstate;
 
 #ifdef USE_PEDANTIC_WRITE
+	const char *func_id= RNA_function_identifier(func);
 	/* testing, for correctness, not operator and not draw function */
-	const short is_readonly= strstr("draw", RNA_function_identifier(func)) || !RNA_struct_is_a(ptr->type, &RNA_Operator);
+	const short is_readonly= strstr("draw", func_id) || strstr("render", func_id) || !RNA_struct_is_a(ptr->type, &RNA_Operator);
 #endif
 
 	py_class= RNA_struct_py_type_get(ptr->type);

source/blender/python/intern/bpy_rna.h

@@ -108,4 +108,6 @@ PyObject *pyrna_py_from_array_index(BPy_PropertyArrayRNA *self, PointerRNA *ptr,
 PyObject *pyrna_math_object_from_array(PointerRNA *ptr, PropertyRNA *prop);
 int pyrna_array_contains_py(PointerRNA *ptr, PropertyRNA *prop, PyObject *value);
 
+int pyrna_write_check(void);
+
 #endif