Issue 6183 - Reduce "Receiving end does not exist" exceptions

lib/filterComposer.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 303 matching lines @@
   });
 });
 
 port.on("composer.quoteCSS", (message, sender) =>
 {
   return quoteCSS(message.CSS);
 });
 
 ext.pages.onLoading.addListener(page =>
 {
-  // A newly opened tab that is still loading has its URL set to about:blank
-  if (/^https?:/.test(page.url.protocol))
-    page.sendMessage({type: "composer.content.finished"});
+  // When tabs start loading we send them a message to ensure that the state
+  // of the "block element" tool is reset. This is necessary since Firefox will
+  // sometimes cache the state of a tab when the user navigates back / forward,
+  // which includes the state of the "block element" tool.
+  // Since sending this message will often fail (e.g. for new tabs which have
+  // just been opened) we catch and ignore any exception thrown.
+  browser.tabs.sendMessage(
+    page.id, {type: "composer.content.finished"}
+  ).catch(() => {});
 });