utils: Remove signal handler item on object destruction

If we're tracking an object destruction and that object gets destroyed,
we need to remove the signals handler storage item, or we'd end up
trying to disconnect from it again when destroying the signals handler.

Closes: #2270

utils.js

@@ -110,6 +110,20 @@ const BasicHandler = class DashToDockBasicHandler {
         (this._storage[label] || []).forEach(item => this._unblock(item));
     }
 
+    _removeByItem(item) {
+        Object.getOwnPropertySymbols(this._storage).forEach(label =>
+            (this._storage[label] = this._storage[label].filter(it => {
+                if (it === item)
+                    return false;
+                if (it.length === item.length &&
+                    it.every((_, idx) => it[idx] === item[idx])) {
+                    this._remove(item);
+                    return false;
+                }
+                return true;
+            })));
+    }
+
     // Virtual methods to be implemented by subclass
 
     /**
@@ -168,15 +182,27 @@ export class GlobalSignalsHandler extends BasicHandler {
                 `found in ${object.constructor.name}`);
         }
 
+        const item = [object];
+        const isDestroy = event === 'destroy';
+        const isParentObject = object === this._parentObject;
+
+        if (isDestroy && !isParentObject) {
+            const originalCallback = callback;
+            callback = () => {
+                this._removeByItem(item);
+                originalCallback();
+            };
+        }
         const id = connector.call(object, event, callback);
+        item.push(id);
 
-        if (event === 'destroy' && object === this._parentObject) {
+        if (isDestroy && isParentObject) {
             this._parentObject.disconnect(this._destroyId);
             this._destroyId =
                 this._parentObject.connect('destroy', () => this.destroy());
         }
 
-        return [object, id];
+        return item;
     }
 
     _remove(item) {