Not a perfect solution but I fixed it like this:
If IsDBNull(DataGridView1.CurrentCell.Value) Then
DataGridView1.CurrentCell.Value = "X"
ElseIf DataGridView1.CurrentCell.Value = "X" Then
DataGridView1.CurrentCell.Value = " "
ElseIf DataGridView1.CurrentCell.Value = " " Then
DataGridView1.CurrentCell.Value = "X"
End If