Improve error management for label printing (#588)

- Handle empty label
- Handle empty printer

lib/api_form.dart

@@ -982,6 +982,7 @@ Future<void> launchApiForm(
       Map<String, dynamic> modelData = const {},
       String method = "PATCH",
       Function(Map<String, dynamic>)? onSuccess,
+      bool Function(Map<String, dynamic>)? validate,
       Function? onCancel,
       IconData icon = TablerIcons.device_floppy
     }) async {
@@ -1071,6 +1072,7 @@ Future<void> launchApiForm(
       formFields,
       method,
       onSuccess: onSuccess,
+      validate: validate,
       fileField: fileField,
       icon: icon,
     ))
@@ -1088,6 +1090,7 @@ class APIFormWidget extends StatefulWidget {
       {
         Key? key,
         this.onSuccess,
+        this.validate,
         this.fileField = "",
         this.icon = TablerIcons.device_floppy,
       }
@@ -1111,6 +1114,8 @@ class APIFormWidget extends StatefulWidget {
 
   final Function(Map<String, dynamic>)? onSuccess;
 
+  final bool Function(Map<String, dynamic>)? validate;
+
   @override
   _APIFormWidgetState createState() => _APIFormWidgetState();
 
@@ -1401,6 +1406,12 @@ class _APIFormWidgetState extends State<APIFormWidget> {
         }
       }
     }
+    
+    final bool isValid = widget.validate?.call(data) ?? true;
+
+    if (!isValid) {
+      return;
+    }
 
     // Run custom onSuccess function
     var successFunc = widget.onSuccess;