Fix possible NPE when opening chat editor (#278867)

Fix #278790

Author: roblourens

src/vs/workbench/contrib/chat/browser/chatSessions/chatSessionTracker.ts

@@ -49,10 +49,10 @@ export class ChatSessionTracker extends Disposable {
 				return;
 			}
 
-			const editor = e.editor as ChatEditorInput;
+			const editor = e.editor;
 			const sessionType = editor.getSessionType();
 
-			const model = this.chatService.getSession(editor.sessionResource!);
+			const model = editor.sessionResource && this.chatService.getSession(editor.sessionResource);
 			if (model) {
 				this.chatSessionsService.registerModelProgressListener(model, () => {
 					this.chatSessionsService.notifySessionItemsChanged(sessionType);

src/vs/workbench/contrib/chat/browser/chatSessions/common.ts

@@ -25,7 +25,7 @@ export type ChatSessionItemWithProvider = IChatSessionItem & {
 	hideRelativeTime?: boolean;
 };
 
-export function isChatSession(schemes: readonly string[], editor?: EditorInput): boolean {
+export function isChatSession(schemes: readonly string[], editor?: EditorInput): editor is ChatEditorInput {
 	if (!(editor instanceof ChatEditorInput)) {
 		return false;
 	}