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/refreshable_state.dart

@@ -64,9 +64,12 @@ abstract class RefreshableState<T extends StatefulWidget> extends State<T> with
 
   // Update current tab selection
   void onTabSelectionChanged(int index) {
-    setState(() {
-      tabIndex = index;
-    });
+
+    if (mounted) {
+      setState(() {
+        tabIndex = index;
+      });
+    }
   }
 
   @override
@@ -87,6 +90,10 @@ abstract class RefreshableState<T extends StatefulWidget> extends State<T> with
 
   Future<void> refresh(BuildContext context) async {
 
+    if (!mounted) {
+      return;
+    }
+
     setState(() {
       loading = true;
     });