Below is my flutter TextEditingController

TextEditingController displayName = TextEditingController();

when fetching document from firebase storage and assiging to the above controller, it throws an error called

Exception has occurred.
_TypeError (type 'String' is not a subtype of type 'TextEditingController')

FirebaseFirestore fireStoreRef = FirebaseFirestore.instance;

await fireStoreRef
        .collection("profiles")
        .doc(widget.userId)
        .get()
        .then((DocumentSnapshot doc) {
      final data = doc.data() as Map<String, dynamic>;
      setState(() {
        displayName = data["displayName"] ?? '';
      });
    });

Can you please help me, what wrong i did here,