Update editor to catch targetChild null

lib/models/documents/history.dart

@@ -89,7 +89,7 @@ class History {
       return false;
     }
     Delta delta = source.removeLast();
-    Delta base = doc.toDelta();
+    Delta base = Delta.from(doc.toDelta());
     Delta inverseDelta = delta.invert(base);
     dest.add(inverseDelta);
     this.lastRecorded = 0;

lib/widgets/editor.dart

@@ -890,7 +890,9 @@ class RenderEditableContainerBox extends RenderBox
       }
       targetChild = childAfter(targetChild);
     }
-    assert(targetChild != null);
+    if (targetChild == null) {
+      throw ('targetChild should not be null');
+    }
     return targetChild;
   }