Use setRenderer instead of ctor

Just in case something hangs onto MouseHelper

src/Terminal.ts

@@ -473,7 +473,7 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
         if (this._theme) {
           this.renderer.setTheme(this._theme);
         }
-        this.mouseHelper = new MouseHelper(this.renderer);
+        this.mouseHelper.setRenderer(this.renderer);
         break;
       case 'scrollback':
         this.buffers.resize(this.cols, this.rows);

src/utils/MouseHelper.ts

@@ -9,6 +9,10 @@ import { IRenderer } from '../renderer/Types';
 export class MouseHelper {
   constructor(private _renderer: IRenderer) {}
 
+  public setRenderer(renderer: IRenderer): void {
+    this._renderer = renderer;
+  }
+
   public static getCoordsRelativeToElement(event: {pageX: number, pageY: number}, element: HTMLElement): [number, number] {
     // Ignore browsers that don't support MouseEvent.pageX
     if (event.pageX === null || event.pageX === undefined) {