Fix manager isDisposed is not set (jupyterlab#15997)

* Fix manager isDisposed is not set

* Check the current code is buggy

* Re apply the fix and lint

* Lint the code

packages/services/src/basemanager.ts

@@ -86,6 +86,7 @@ export abstract class BaseManager implements IManager {
     if (this.isDisposed) {
       return;
     }
+    this._isDisposed = true;
     this._disposed.emit(undefined);
     Signal.clearData(this);
   }

packages/services/test/kernelspec/manager.spec.ts

@@ -59,6 +59,17 @@ describe('kernel/manager', () => {
       });
     });
 
+    describe('#isDisposed', () => {
+      it('should be false when instantiating', () => {
+        expect(manager.isDisposed).toBe(false);
+      });
+
+      it('should true when disposing', () => {
+        manager.dispose();
+        expect(manager.isDisposed).toBe(true);
+      });
+    });
+
     describe('#serverSettings', () => {
       it('should get the server settings', () => {
         manager.dispose();