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_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