Fix T81674: LibOverride: 'Define Override' UI operation can crash.

Case where RNA is not able to generate a proper path for a property was
not taken into account properly.

source/blender/editors/interface/interface_ops.c

@@ -558,6 +558,13 @@ static int override_type_set_button_exec(bContext *C, wmOperator *op)
 
   IDOverrideLibraryPropertyOperation *opop = RNA_property_override_property_operation_get(
       CTX_data_main(C), &ptr, prop, operation, index, true, NULL, &created);
+
+  if (opop == NULL) {
+    /* Sometimes e.g. RNA cannot generate a path to the given property. */
+    BKE_reportf(op->reports, RPT_WARNING, "Failed to create the override operation");
+    return OPERATOR_CANCELLED;
+  }
+
   if (!created) {
     opop->operation = operation;
   }