Bug 1903980 - [devtools] Better handle short lived iframes. r=devtools-reviewers,nchevobbe

WindowGlobalTarget actors are currently being destroyed via
the docshell listeners registered by the WindowGlobalTargetActor class (_watchDocShells).
This is a bit unexpected as the WindowGlobal Watcher Class is meant to
handle the lifecycle (creation and destruction) of these actors.
On top of that, this docshell listener code (_watchDocShells) isn't really
meant to be executed. It's main goal is to implement the non-EFT(EveryFrameTarget)
codepath, where the WindowGlobalTarget actor may be focusing on more than one
WindowGlobal/document instance.

The situation here is that this code throws on creation as well as destruction,
leading to a broken target actor still registered in TargetRegistry/ContentProcessWatcherRegistry.

Differential Revision: https://phabricator.services.mozilla.com/D214755

devtools/server/actors/targets/window-global.js

@@ -868,6 +868,13 @@ class WindowGlobalTargetActor extends BaseTargetActor {
       return;
     }
 
+    // This method is called asynchronously and the document may have been destroyed in the meantime.
+    // In such case, automatically destroy the target actor.
+    if (this.docShell.isBeingDestroyed()) {
+      this.destroy();
+      return;
+    }
+
     // In child processes, we watch all docshells living in the process.
     Services.obs.addObserver(this, "webnavigation-create");
     Services.obs.addObserver(this, "webnavigation-destroy");
@@ -1743,6 +1750,12 @@ class DebuggerProgressListener {
   }
 
   unwatch(docShell) {
+    // If the docshell is being destroyed, we won't be able to retrieve its related window object,
+    // which is the key ingredient for all cleanup operations done in this method.
+    if (docShell.isBeingDestroyed()) {
+      return;
+    }
+
     const docShellWindow = docShell.domWindow;
     if (!this._watchedDocShells.has(docShellWindow)) {
       return;