Bug 1639997 - Have webrtcUI handle the case where the selectedTab is set to null. r=pbz

It's possible for gBrowser.selectedTab to be set to null. This should be a no-op, but this
part of webrtcUI that tabbrowser calls doesn't handle the case, and causes an exception to
be thrown. This patch allows the no-op case to occur properly.

Differential Revision: https://phabricator.services.mozilla.com/D76736

browser/modules/webrtcUI.jsm

@@ -660,6 +660,10 @@ var webrtcUI = {
   },
 
   shouldShowSharedTabWarning(tab) {
+    if (!tab || !tab.linkedBrowser) {
+      return false;
+    }
+
     let browser = tab.linkedBrowser;
     // We want the user to be able to switch to one tab after starting
     // to share their window or screen. The presumption here is that