Bug 1698842 - [devtools] Avoid notifying about already destroyed targets from TargetCommand. r=nchevobbe

This helps browser_toolbox_backward_forward_navigation.js passing with fission enabled.

Differential Revision: https://phabricator.services.mozilla.com/D108648

devtools/shared/commands/target/target-command.js

@@ -176,6 +176,10 @@ class TargetCommand extends EventEmitter {
       targetFrontsSet.delete(targetFront);
     }
 
+    if (this.isDestroyed() || targetFront.isDestroyedOrBeingDestroyed()) {
+      return;
+    }
+
     // Then, once the target is attached, notify the target front creation listeners
     await this._createListeners.emitAsync(targetType, {
       targetFront,