[Web UI] Quick fix for FireFox resize event bug (#2475)

Event objects in FireFox appear to get garbage collected sooner than in other browsers. Prevent unintended null reference at `event.currentTarget` by not passing the event object into the async debounce handler.

CHANGELOG.md

@@ -7,6 +7,9 @@ Versioning](http://semver.org/spec/v2.0.0.html).
 
 ## Unreleased
 
+### Fixed
+- Fixed a web-ui bug causing the app to crash on window resize in FireFox
+
 ### [5.0.0] - 2018-11-30
 
 ### Added

dashboard/src/components/partials/ToolbarMenu/Menu.js

@@ -69,7 +69,7 @@ class ToolbarMenu extends React.PureComponent {
   };
 
   componentWillUnmount() {
-    this.handleWindowResize.clear();
+    this.updateButtonsWidth.clear();
   }
 
   handleOverflowButtonResize = rect => {
@@ -92,8 +92,12 @@ class ToolbarMenu extends React.PureComponent {
     });
   };
 
-  handleWindowResize = debounce(ev => {
-    const newWidth = ev.currentTarget.innerWidth;
+  handleWindowResize = event => {
+    this.updateButtonsWidth(event.currentTarget);
+  };
+
+  updateButtonsWidth = debounce(currentTarget => {
+    const newWidth = currentTarget.innerWidth;
     const oldWidth = this.windowWidth || 0;
 
     // If the window grew in size and the toolbar menu isn't configured to fill