Add validation to prevent adding closed FormGroupCubits as subforms

lib/src/form_group_cubit/form_group_cubit.dart

@@ -206,7 +206,14 @@ class FormGroupCubit extends Cubit<FormGroupState> with Disposable {
 
   /// Adds a subform to the current form.
   /// If [form] was already added as a subform this is a noop.
+  /// If [form] is closed this will throw a [StateError].
   void addSubform(FormGroupCubit form) {
+    if (form.isClosed) {
+      throw StateError(
+        'Cannot add a closed form',
+      );
+    }
+
     emit(
       FormGroupState(
         wasModified: state.wasModified,

test/src/form_group_cubit/form_group_cubit_test.dart

@@ -486,6 +486,20 @@ void main() {
         },
         expect: () => <dynamic>[],
       );
+
+      blocTest<FormGroupCubit, FormGroupState>(
+        'throws if form is closed',
+        build: () => form,
+        setUp: () => subform.close(),
+        act: (cubit) {
+          cubit.addSubform(subform);
+        },
+        errors: () => [
+          predicate(
+            (e) => e is StateError && e.message == 'Cannot add a closed form',
+          ),
+        ],
+      );
     });
 
     group('validateAll', () {