Dart/Flutter & Firebase RTDB - How to access the nested Map retrieved from RTDB

Question

I'm working with data from Firebase RTDB. While I've successfully gotten the whole data from Firestore through this piece:

  // GETTING ALL THE DATA FROM RTDB - MAP
  void readSchool() async {
    await realtime.once().then((DataSnapshot snapshot) {
      setState(() {
        schoolList = snapshot.value;
      });
    });
    print('Data: $schoolList');
  }

This is my RTDB data structure:

The Map I got from RTDB came out as nested Map (Map inside Map).

    Data: 
{
Kỹ thuật và Công nghệ: 
{
 color1: 0xfffaaca8, color2: 0xffb06ab3, 
 description: Nghiên cứu và sáng tạo các sản phẩm công nghệ cao,...,
   oisp: {
      imgcard: imgurl1, 
      category: KỸ THUẬT VÀ CÔNG NGHỆ, 
      title: Quốc tế Đại học Bách khoa TP.HCM, 
      location_s: Quận 10, Thành phố Hồ Chí Minh
   }
}, 

Kinh tế và Quản lý: 
{
 color1: 0xffec6f66, color2: 0xfff3a183, 
 description: Tổ chức điều phối các hoạt động kinh tế, quản lý, tài chính,..., 
 ueh: 
   {
    imgcard: imgurl2, 
    category: KINH TẾ VÀ QUẢN LÝ, 
    title: Đại học Kinh tế TP.HCM, 
    location_s: Quận 3, Thành phố Hồ Chí Minh}
   }
}

Apparently, accessing a specific data using normal syntax such as schoolList['Kỹ thuật và Công nghệ']['oisp'] doesn't trigger an analysis call of error but doesn't return a value neither.

Please help me access a specific data inside the data I retrieved from RTDB. Thank you!

---

I am trying to create some objects named HomeCard here:

  // LIST OF ALL SCHOOL
  Map schoolList;

  // LIST OF FEATURED SCHOOL FROM FEATURED/RECOMMEND
  List featuredSchool = [];

  // LIST OF CATEGORIES FROM INFORMATION CATEGORIZE/LIST
  List categories = [];

  void homeCard() {
for (String category in categories) {
  // ADD TO LIST
  for (String school in featuredSchool) {
    if (schoolList[category][school] as String != null) {
        featuredCard.add(
          // CARD TEMPLATE
          HomeCard(
            // FROM UNIVERSITY/INFORMATION/SCHOOL/[RECOMMENDED SCHOOL]/'TITLE'
            name: schoolList[category][school]['title'] as String,

            // FROM UNIVERSITY/INFORMATION/SCHOOL/[RECOMMENDED SCHOOL]/'CATEGORY'
            category: schoolList[category][school]['category'] as String,

            // FROM UNIVERSITY/INFORMATION/SCHOOL/[RECOMMENDED SCHOOL]/'IMGCARD'
            networkImage:
                NetworkImage(schoolList[category][school]['imgcard'] as String),

            // CARD FUNCTION
            function: () {},
          ),
        );
        String name = schoolList[category][school]['title'] as String;
        print(name);
    }
  }
}

}

but it outputs an empty list