Don't try to flash messages on null host page

This was causing a race condition on app startup. Now, the worst that
happens is the flash message shows in the sidebar.

h/js/flash.coffee

@@ -20,7 +20,7 @@ class FlashProvider
       if msgs.length
         msg = msgs.shift()
         unless q then [q, msg] = msg
-        if annotator.isOpen()
+        if annotator.isOpen() or not annotator.host
           notice = Annotator.showNotification msg, q
           @timeout = this._wait =>
             # work around Annotator.Notification not removing classes
@@ -87,4 +87,4 @@ angular.module('h.flash', ['ngResource'])
 .factory('flashInterceptor', flashInterceptor)
 .config(['$httpProvider', ($httpProvider) ->
   $httpProvider.interceptors.push 'flashInterceptor'
-])
\ No newline at end of file
+])