Assert initialValue is null or controller is null for FormBuilderTextField. Closes #258

Author: danvick

lib/src/fields/form_builder_text_field.dart

@@ -87,7 +87,8 @@ class FormBuilderTextField extends StatefulWidget {
     this.showCursor,
     this.onSaved,
     this.onTap,
-  }) : super(key: key);
+  })  : assert(initialValue == null || controller == null),
+        super(key: key);
 
   @override
   FormBuilderTextFieldState createState() => FormBuilderTextFieldState();