Bug 1686748 - [devtools] Fix WalkerActor exception on server side target switching. r=nchevobbe

When using JSWindowActor based target, each target actor is destroyed on each reload/navigation.
This code in walker actor is registering a DOMContentLoaded listener which
only fires after the target is already destroyed. This leads to exceptions.

Differential Revision: https://phabricator.services.mozilla.com/D114367

devtools/server/actors/inspector/walker.js

@@ -2472,6 +2472,10 @@ var WalkerActor = protocol.ActorClassWithSpec(walkerSpec, {
   },
 
   onFrameLoad: function({ window, isTopLevel }) {
+    // By the time we receive the DOMContentLoaded event, we might have been destroyed
+    if (this._destroyed) {
+      return;
+    }
     const { readyState } = window.document;
     if (readyState != "interactive" && readyState != "complete") {
       // The document is not loaded, so we want to register to fire again when the