Move syncBellSound after helperContainer init

src/Terminal.ts

@@ -593,9 +593,6 @@ export class Terminal extends EventEmitter implements ITerminal, IInputHandlingT
     this.viewportScrollArea.classList.add('xterm-scroll-area');
     this.viewportElement.appendChild(this.viewportScrollArea);
 
-    // preload audio
-    this.syncBellSound();
-
     this._mouseZoneManager = new MouseZoneManager(this);
     this.on('scroll', () => this._mouseZoneManager.clearAll());
     this.linkifier.attachToDom(this._mouseZoneManager);
@@ -625,6 +622,9 @@ export class Terminal extends EventEmitter implements ITerminal, IInputHandlingT
     this.helperContainer.appendChild(this.charSizeStyleElement);
     this.charMeasure = new CharMeasure(document, this.helperContainer);
 
+    // Preload audio, this relied on helperContainer
+    this.syncBellSound();
+
     // Performance: Add viewport and helper elements from the fragment
     this.element.appendChild(fragment);