Fix: snackbar is sometimes uninitialized

Sometimes the hideUpdatePrompt() method is called before setUp() is
called, which results in the snackbar being uninitialized and
hideUpdatePrompt() fails.

This is the second attempt to fix this issue. The first attempt was
reverted since it broke the feature.

Bug: twpowertools:36
Change-Id: I7114c5fb7be293248a8303a3d5e5120f14573101
diff --git a/src/contentScripts/communityConsole/autoRefresh.js b/src/contentScripts/communityConsole/autoRefresh.js
index 38e83f3..edf463c 100644
--- a/src/contentScripts/communityConsole/autoRefresh.js
+++ b/src/contentScripts/communityConsole/autoRefresh.js
@@ -140,7 +140,7 @@
   }
 
   hideUpdatePrompt() {
-    this.snackbar.classList.add('TWPT-hidden');
+    if (this.snackbar) this.snackbar.classList.add('TWPT-hidden');
     document.title = document.title.replace('[!!!] ', '');
     this.isUpdatePromptShown = false;
   }