Merge pull request #1355 from n-bernat/fix/unregister-field-assert-me…

…ssage

Add a message to an assertion

lib/src/form_builder.dart

@@ -221,7 +221,11 @@ class FormBuilderState extends State<FormBuilder> {
   }
 
   void unregisterField(String name, FormBuilderFieldState field) {
-    assert(_fields.containsKey(name));
+    assert(
+      _fields.containsKey(name),
+      'Failed to unregister a field. Make sure that all field names in a form are unique.',
+    );
+
     // Only remove the field when it is the one registered.  It's possible that
     // the field is replaced (registerField is called twice for a given name)
     // before unregisterField is called for the name, so just emit a warning