Quick input shouldn't completely break if a ThemeIcon is provided

Part of #72489

src/vs/workbench/browser/parts/quickinput/quickInput.ts

@@ -255,7 +255,7 @@ class QuickInput implements IQuickInput {
 			this.ui.leftActionBar.clear();
 			const leftButtons = this.buttons.filter(button => button === backButton);
 			this.ui.leftActionBar.push(leftButtons.map((button, index) => {
-				const action = new Action(`id-${index}`, '', button.iconClass || getIconClass(button.iconPath!), true, () => {
+				const action = new Action(`id-${index}`, '', button.iconClass || getIconClass(button.iconPath), true, () => {
 					this.onDidTriggerButtonEmitter.fire(button);
 					return Promise.resolve(null);
 				});
@@ -265,7 +265,7 @@ class QuickInput implements IQuickInput {
 			this.ui.rightActionBar.clear();
 			const rightButtons = this.buttons.filter(button => button !== backButton);
 			this.ui.rightActionBar.push(rightButtons.map((button, index) => {
-				const action = new Action(`id-${index}`, '', button.iconClass || getIconClass(button.iconPath!), true, () => {
+				const action = new Action(`id-${index}`, '', button.iconClass || getIconClass(button.iconPath), true, () => {
 					this.onDidTriggerButtonEmitter.fire(button);
 					return Promise.resolve(null);
 				});

src/vs/workbench/browser/parts/quickinput/quickInputUtils.ts

@@ -11,7 +11,10 @@ import { IdGenerator } from 'vs/base/common/idGenerator';
 const iconPathToClass = {};
 const iconClassGenerator = new IdGenerator('quick-input-button-icon-');
 
-export function getIconClass(iconPath: { dark: URI; light?: URI; }) {
+export function getIconClass(iconPath: { dark: URI; light?: URI; } | undefined): string | undefined {
+	if (!iconPath) {
+		return undefined;
+	}
 	let iconClass: string;
 
 	const key = iconPath.dark.toString();