Fix spurious error message in dev console

Ports can be disconnected under certain conditions,
and in such case runtime.lastError must be accessed
to avoid the extensions framework from emitting
error messages in the console.

platform/common/vapi-client.js

@@ -97,9 +97,12 @@ vAPI.messaging = {
     //   as world-ending, i.e. stay around. Except for embedded frames.
 
     disconnectListener: function() {
+        void browser.runtime.lastError;
         this.port = null;
         if ( window !== window.top ) {
             vAPI.shutdown.exec();
+        } else {
+            this.destroyPort();
         }
     },
     disconnectListenerBound: null,
@@ -129,11 +132,10 @@ vAPI.messaging = {
     messageListenerBound: null,
 
     canDestroyPort: function() {
-        return this.pending.size === 0 &&
-            (
-                this.extensions.length === 0 ||
-                this.extensions.every(e => e.canDestroyPort())
-            );
+        return this.pending.size === 0 && (
+            this.extensions.length === 0 ||
+            this.extensions.every(e => e.canDestroyPort())
+        );
     },
 
     mustDestroyPort: function() {