catch missing src in img

aqt/editor.py

@@ -296,8 +296,13 @@ def _filterHTML(html):
             tag.replaceWithChildren()
     # turn file:/// links into relative ones
     for tag in doc("img"):
+        try:
             if tag['src'].lower().startswith("file://"):
                 tag['src'] = os.path.basename(tag['src'])
+        except KeyError:
+            # for some bizarre reason, mnemosyne removes src elements
+            # from missing media
+            pass
     # strip superfluous elements
     for elem in "html", "head", "body", "meta":
         for tag in doc(elem):