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

lib/filterComposer.js

Index: lib/filterComposer.js
diff --git a/lib/filterComposer.js b/lib/filterComposer.js
index 3759ca58e9875ccaa175310f6bd9dab52285b7dd..e55a4fc6e19d4993d4a1c8c737ae1e5a6e4859a0 100644
--- a/lib/filterComposer.js
+++ b/lib/filterComposer.js
@@ -321,7 +321,13 @@ port.on("composer.quoteCSS", (message, sender) =>
 
 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(() => {});
 });