Bug 1615418 - Do not throw when calling watch/unwatchFront on destroyed Fronts r=daisuke,ochameau

Depends on D62893

The issue here is that we are trying to destroy the workers-listener after the target was destroyed,
and calling unwatchFront on a destroyed Front throws.

Most of the fronts monitored in workers-listener are handled by the watchFront API, so properly adding
onDestroyed handlers fixes some issues. However the rootFront cannot be handled with a similar pattern
at the moment.

In general, I think making watchFront/unwatchFront safer to call makes sense, but I could also check
if the rootFront is already destroyed in workers-listener's destroy

Differential Revision: https://phabricator.services.mozilla.com/D62894

--HG--
extra : moz-landing-system : lando

devtools/shared/protocol/Front.js

@@ -125,6 +125,15 @@ class Front extends Pool {
    *        The function is called with the same argument than onAvailable.
    */
   watchFronts(typeName, onAvailable, onDestroy) {
+    if (!this.actorID) {
+      // The front was already destroyed, bail out.
+      console.error(
+        `Tried to call watchFronts for the '${typeName}' type on an ` +
+          `already destroyed front '${this.typeName}'.`
+      );
+      return;
+    }
+
     if (onAvailable) {
       // First fire the callback on already instantiated fronts
       for (const front of this.poolChildren()) {
@@ -147,6 +156,15 @@ class Front extends Pool {
    * See `watchFronts()` for documentation of the arguments.
    */
   unwatchFronts(typeName, onAvailable, onDestroy) {
+    if (!this.actorID) {
+      // The front was already destroyed, bail out.
+      console.error(
+        `Tried to call unwatchFronts for the '${typeName}' type on an ` +
+          `already destroyed front '${this.typeName}'.`
+      );
+      return;
+    }
+
     if (onAvailable) {
       this._frontCreationListeners.off(typeName, onAvailable);
     }