Only update viewlet switcher if it has already been rendered (#28568)

src/vs/workbench/browser/parts/activitybar/activitybarPart.ts

@@ -293,6 +293,11 @@ export class ActivitybarPart extends Part implements IActivityBarService {
 	}
 
 	private updateViewletSwitcher() {
+		if (!this.viewletSwitcherBar) {
+			// We have not been rendered yet so there is nothing to update.
+			return;
+		}
+
 		let viewletsToShow = this.getPinnedViewlets();
 
 		// Always show the active viewlet even if it is marked to be hidden