Add null check to TerminalPanel.focus

Fixes #22368

src/vs/workbench/parts/terminal/electron-browser/terminalPanel.ts

@@ -137,7 +137,10 @@ export class TerminalPanel extends Panel {
 	}
 
 	public focus(): void {
-		this._terminalService.getActiveInstance().focus(true);
+		const activeInstance = this._terminalService.getActiveInstance();
+		if (activeInstance) {
+			activeInstance.focus(true);
+		}
 	}
 
 	private _attachEventListeners(): void {