Fix: Enable "Add" buttons after removing child that can only be added

once (for Step Builder and Review Step).

problem_builder/public/js/mentoring_with_steps_edit.js

@@ -26,8 +26,13 @@ function MentoringWithStepsEdit(runtime, element) {
         $buttons.on('click', disableButton);
     };
 
+    var updateButtons = function() {
         initButtons('sb-review-step');
+    };
 
     ProblemBuilderUtil.transformClarifications(element);
-    StudioEditableXBlockMixin(runtime, element);
+
+    updateButtons();
+    runtime.listenTo('deleted-child', updateButtons);
+
 }

problem_builder/public/js/review_step_edit.js

@@ -27,8 +27,13 @@ function ReviewStepEdit(runtime, element) {
         $buttons.on('click', disableButton);
     };
 
+    var updateButtons = function() {
         initButtons('pb-message');
+    };
 
     ProblemBuilderUtil.transformClarifications(element);
-    StudioEditableXBlockMixin(runtime, element);
+
+    updateButtons();
+    runtime.listenTo('deleted-child', updateButtons);
+
 }