Merge pull request #1504 from areyeshot/develop

fix #1376

core/extensions.js

@@ -74,6 +74,9 @@ Blockly.Extensions.register = function(name, initFn) {
  *     registered.
  */
 Blockly.Extensions.registerMixin = function(name, mixinObj) {
+  if (!goog.isObject(mixinObj)){
+    throw new Error('Error: Mixin "' + name + '" must be a object');
+  }
   Blockly.Extensions.register(name, function() {
     this.mixin(mixinObj);
   });