Fix js error about viewState being undefined

In change 355716 it was claimed that:
```
// viewState is set by gr-router in handleChangeRoute method and is
// never set to undefined
```

That claim is wrong, see `this.changeViewModel.setState(undefined);`
in the `setState()` method.

Release-Notes: skip
Change-Id: Ibb2bb27494ad100c235306313804b7947846c6e8

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

diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
index 6c1b681..d4627e2 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
@@ -2079,9 +2079,7 @@
 
   // Private but used in tests.
   viewStateChanged() {
-    // viewState is set by gr-router in handleChangeRoute method and is never
-    // set to undefined
-    assertIsDefined(this.viewState, 'viewState');
+    if (!this.viewState) return;
 
     if (this.isChangeObsolete()) {
       // Tell the app element that we are not going to handle the new change