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/api_form.dart

@@ -18,6 +18,7 @@ import "package:inventree/widget/fields.dart";
 import "package:inventree/l10.dart";
 
 import "package:flutter/material.dart";
+import "package:inventree/widget/progress.dart";
 import "package:inventree/widget/snacks.dart";
 
 
@@ -859,7 +860,9 @@ Future<void> launchApiForm(
 
   if (url.isNotEmpty) {
 
+    showLoadingOverlay(context);
     var options = await InvenTreeAPI().options(url);
+    hideLoadingOverlay();
 
     // Invalid response from server
     if (!options.isValid()) {
@@ -902,7 +905,7 @@ Future<void> launchApiForm(
     field.definition = extractFieldDefinition(serverFields, field.lookupPath);
 
     // Skip fields with empty definitions
-    if (field.definition.isEmpty) {
+    if (url.isNotEmpty && field.definition.isEmpty) {
       print("Warning: Empty field definition for field '${fieldName}'");
     }
 
@@ -987,8 +990,6 @@ class _APIFormWidgetState extends State<APIFormWidget> {
 
   List<String> nonFieldErrors = [];
 
-  Function(Map<String, dynamic>)? onSuccess;
-
   bool spacerRequired = false;
 
   List<Widget> _buildForm() {
@@ -1102,20 +1103,25 @@ class _APIFormWidgetState extends State<APIFormWidget> {
     }
 
     if (widget.method == "POST") {
+
+      showLoadingOverlay(context);
       final response =  await InvenTreeAPI().post(
         widget.url,
         body: data,
         expectedStatusCode: null
       );
+      hideLoadingOverlay();
 
       return response;
 
     } else {
+      showLoadingOverlay(context);
       final response = await InvenTreeAPI().patch(
         widget.url,
         body: data,
         expectedStatusCode: null
       );
+      hideLoadingOverlay();
 
       return response;
     }
@@ -1259,7 +1265,7 @@ class _APIFormWidgetState extends State<APIFormWidget> {
     }
 
     // Run custom onSuccess function
-    var successFunc = onSuccess;
+    var successFunc = widget.onSuccess;
 
     // An "empty" URL means we don't want to submit the form anywhere
     // Perhaps we just want to process the data?
@@ -1398,4 +1404,4 @@ class _APIFormWidgetState extends State<APIFormWidget> {
     );
 
   }
-}
+}