[R] Fix call/ring overlay

There were regressions in the form of JS errors in Filmstrip introduced
when the call/ring overlay was rewritten in React.

react/features/filmstrip/middleware.js

@@ -17,7 +17,7 @@ declare var APP: Object;
 MiddlewareRegistry.register(({ getState }) => next => action => {
     switch (action.type) {
     case SET_CALL_OVERLAY_VISIBLE:
-        if (typeof APP === 'undefined') {
+        if (typeof APP !== 'undefined') {
             const oldValue
                 = Boolean(getState()['features/jwt'].callOverlayVisible);
             const result = next(action);
@@ -25,7 +25,14 @@ MiddlewareRegistry.register(({ getState }) => next => action => {
                 = Boolean(getState()['features/jwt'].callOverlayVisible);
 
             oldValue === newValue
-                || Filmstrip.toggleFilmstrip(!newValue, false);
+
+                // FIXME The following accesses the private state filmstrip of
+                // Filmstrip. It is written with the understanding that
+                // Filmstrip will be rewritten in React and, consequently, will
+                // not need the middleware implemented here, Filmstrip.init, and
+                // UI.start.
+                || (Filmstrip.filmstrip
+                    && Filmstrip.toggleFilmstrip(!newValue, false));
 
             return result;
         }