Display overlay screen for blocking operations (#186)

* Catch state error in homepage widget

* Add flutter_overlay_loader lib

- Displays an overlay screen to indicate blocking operation

* Wrap blocking widget transitions in a loading overlay

- Prevents user from doing other things while loading
- Shows the user that something is happening

* Linting fixes

* Show overlay when uploading attachment file

* Show overlay when downloading file also

* Show overlay when loading or submitting API forms

- Major improvements to usability "feel"

* UI improvements for stock item test results widget

* Fix API_FORM bug

- onSuccess function was not being called

lib/widget/home.dart

@@ -54,11 +54,13 @@ class _InvenTreeHomePageState extends State<InvenTreeHomePage> {
     });
 
     InvenTreeAPI().registerCallback(() {
-      setState(() {
-        // Reload the widget
-      });
-    });
 
+      if (mounted) {
+        setState(() {
+          // Reload the widget
+        });
+      }
+    });
   }
 
   // Index of bottom navigation bar
@@ -192,6 +194,11 @@ class _InvenTreeHomePageState extends State<InvenTreeHomePage> {
       return;
     }
 
+    // Ignore if the widget is no longer active
+    if (!mounted) {
+      return;
+    }
+
     final notifications = await InvenTreeNotification().list();
 
     setState(() {