Don't cause a 500 error in the LMS if get_block unexpectedly returns None

problem_builder/mentoring.py

@@ -236,7 +236,9 @@ class MentoringBlock(XBlock, StepParentMixin, StudioEditableXBlockMixin, StudioC
 
         for child_id in self.children:
             child = self.runtime.get_block(child_id)
-            if not isinstance(child, MentoringMessageBlock):
+            if child is None:  # child should not be None but it can happen due to bugs or permission issues
+                child_content += u"<p>[{}]</p>".format(self._(u"Error: Unable to load child component."))
+            elif not isinstance(child, MentoringMessageBlock):
                 try:
                     if self.is_assessment and isinstance(child, StepMixin):
                         child_fragment = child.render('assessment_step_view', context)

problem_builder/tests/unit/test_mentoring.py

@@ -31,6 +31,20 @@ class TestMentoringBlock(unittest.TestCase):
 
             self.assertFalse(patched_runtime.publish.called)
 
+    def test_does_not_crash_when_get_child_is_broken(self):
+        block = MentoringBlock(MagicMock(), DictFieldData({
+            'children': ['invalid_id'],
+        }), Mock())
+
+        with patch.object(block, 'runtime') as patched_runtime:
+            patched_runtime.publish = Mock()
+            patched_runtime.service().ugettext = lambda str: str
+            patched_runtime.get_block = lambda block_id: None
+
+            fragment = block.student_view(context={})
+
+            self.assertIn('Unable to load child component', fragment.content)
+
 
 @ddt.ddt
 class TestMentoringBlockTheming(unittest.TestCase):