Koenig - Fixed error from plus menu when leaving editor

refs https://github.com/TryGhost/Ghost/issues/9724
- leaving the editor could result in the plus menu being removed from DOM before teardown has fully cleared up the event handlers resulting in an error being thrown in the mousemove handler where we assumed the element is present

lib/koenig-editor/addon/components/koenig-plus-menu.js

@@ -213,7 +213,7 @@ export default Component.extend({
 
     // show the (+) button when the mouse is over a blank P tag
     _handleMousemove(event) {
-        if (!this.showMenu) {
+        if (!this.showMenu && this.element) {
             let {pageX, pageY} = event;
             let editor = this.editor;