fix: fix a crash when adding a broadcast message (#150)

Author: gonfunko

src/variables.js

@@ -117,7 +117,7 @@ export function createVariable(workspace, opt_callback, opt_type) {
 
         var flyout = workspace.isFlyout ? workspace : workspace.getFlyout();
         var variableBlockId = variable.getId();
-        workspace.getToolbox().runAfterRerender(() => {
+        workspace.getToolbox()?.runAfterRerender(() => {
           flyout.setCheckboxState(variableBlockId, true);
         });