Don't set the change to null due to a nav away from change view

It ends up triggering the observers that then try to operate on
a null object, which has its own set of problems.

Change-Id: I781b5871f06b771e367f48469005d952af74dd42

polygerrit-ui/app/elements/gr-change-view.html

@@ -291,8 +291,6 @@ limitations under the License.
         this._changeNum = value.changeNum;
         this._patchNum = value.patchNum;
         if (!this._changeNum) {
-          this._change = null;
-          this._comments = null;
           return;
         }
         this._reload();