Handle quick Cursor detach after mouse up

This timer might fire after the Cursor object has detached from a DOM
element, causing crashes. This will likely not happen in real scenarios,
but the tests are quick enough to trigger this.

kasmweb/core/util/cursor.js

@@ -170,6 +170,10 @@ export default class Cursor {
         // should be visible.
         if (this._captureIsActive()) {
             window.setTimeout(() => {
+                // We might have detached at this point
+                if (!this._target) {
+                    return;
+                }
                 // Refresh the target from elementFromPoint since queued events
                 // might have altered the DOM
                 target = document.elementFromPoint(event.clientX,