Fixed #4206 and #4207 open and close notebook quickly issue #4240 (#4255)

* Fixed  #4206 and #4207

* Check if it makes my PR run tests

* Add isReady to JupyterSessionManager

extensions/notebook/src/jupyter/jupyterSessionManager.ts

@@ -150,11 +150,16 @@ export class JupyterSessionManager implements nb.SessionManager {
 	}
 
 	public shutdownAll(): Promise<void> {
-		return this._sessionManager.shutdownAll();
+		if (this._isReady) {
+			return this._sessionManager.shutdownAll();
+		}
+		return Promise.resolve();
 	}
 
 	public dispose(): void {
-		this._sessionManager.dispose();
+		if (this._isReady) {
+			this._sessionManager.dispose();
+		}
 	}
 }