DropDownButton inside FutureBuilder gets "Null check operator used on a null value" error-CodePudding

I'm trying to create a DropDownButton using the data from a table. I've created a function to return a list of objects which then can be used to convert those to a list of string but I keep getting "Null check operator used on a null value" error. So I tried to remove as many exclamation mark as I can. But I'm still getting the same error.

This is the function I use to get the list of objects from the DatabaseHelper class.

Future<List<Category>> getCategories() async {
  Database? db = await instance.database;
  var categories = await db?.query('Category', orderBy: 'id');

  List<Category> categoryList = categories!.isNotEmpty ? categories.map((c) => 
    Category.fromMap(c)).toList(): [];

  return categoryList;
}

This is the FutureBuilder and the DropDropButton

child: FutureBuilder<List<Category>> (
            future: DatabaseHelper.instance.getCategories(),
            builder: (BuildContext context, AsyncSnapshot<List<Category>> snapshot) {
              if (!snapshot.hasData) {
                return const Text('No data');
              }
              else {
                return DropdownButton(
                  onChanged: (value) {  },
                  items: snapshot.data?.map((category) =>
                      DropdownMenuItem<String>(
                        value: category.categoryName,
                        child: Text(category.categoryName),
                      )
                  ).toList(),
                );
              }
            },
          )

CodePudding user response：

Please change categories!.isNotEmpty to categories?.isNotEmpty

CodePudding user response：

For some reason categories is null, i.e it's not properly fetched from the database. So this is leading to invoke isNotEmpty on a null value i.e null.isNotEmpty which is wrong, so the above error.

To overcome this , dont use the isNotEmpty property

Use something like :

 List<Category> categoryList = categories ? categories.map((c) => 
    Category.fromMap(c)).toList(): [];

CodePudding user response：

Setting the isExpanded to true solves my problem.