Issue 5347 - Check for contextMenus API support

lib/filterComposer.js

Index: lib/filterComposer.js
===================================================================
--- a/lib/filterComposer.js
+++ b/lib/filterComposer.js
@@ -179,31 +179,36 @@
   page.contextMenus.remove(contextMenuItem);
 
   if (typeof filter == "undefined")
     filter = checkWhitelisted(page);
   if (!filter && Prefs.shouldShowBlockElementMenu && readyPages.has(page))
     page.contextMenus.create(contextMenuItem);
 }
 
-FilterNotifier.on("page.WhitelistingStateRevalidate", updateContextMenu);
-
-Prefs.on("shouldShowBlockElementMenu", () =>
+if ("contextMenus" in chrome)
 {
-  ext.pages.query({}, pages =>
+  FilterNotifier.on("page.WhitelistingStateRevalidate", updateContextMenu);
+
+  Prefs.on("shouldShowBlockElementMenu", () =>
   {
-    for (let page of pages)
-      updateContextMenu(page);
+    ext.pages.query({}, pages =>
+    {
+      for (let page of pages)
+        updateContextMenu(page);
+    });
   });
-});
+}
 
 port.on("composer.ready", (message, sender) =>
 {
   readyPages.set(sender.page, null);
-  updateContextMenu(sender.page);
+
+  if ("contextMenus" in chrome)
+    updateContextMenu(sender.page);
 });
 
 port.on("composer.openDialog", (message, sender) =>
 {
   return new Promise(resolve =>
   {
     ext.windows.create({
       url: ext.getURL("composer.html"),