White lines in Grid View, Flutter

I wanted to develop a Sudoku app. I have created a list for all positions in the game field. To display the fields I used a grid view. However, I have the problem that white lines appear in the grid view and I don’t understand why. In the attached image you can see this in more detail.

Here is my code for the grid view:

  GridView.builder(
    itemCount: gameFields.length,
    gridDelegate: const SliverGridDelegateWithFixedCrossAxisCount(
      crossAxisCount: 9,
    ),
    itemBuilder: (context, index) => Container(
      color: Colors.lightGreen,
      child: Center(
        child: Text("${index + 1}"),
      ),
    ),
  );