fixes #1889

blocks.js

@@ -3462,10 +3462,12 @@ BlockMorph.prototype.doRefactorGlobalVar = function (
                 true
             );
             stage.globalBlocks.forEach(function (eachBlock) {
+                if (eachBlock.body) {
                     eachBlock.body.expression.refactorVarInStack(
                         oldName,
                         newName
                     );
+                }
             });
             stage.forAllChildren(function (child) {
                 if (child instanceof SpriteMorph) {