Как устранить исключение NullReferenceException?

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim i, j, n, m, max, min, fm, qm As Integer
        Dim c(,) As Integer
        n = Val(InputBox("Количество строк"))
        m = Val(InputBox("Количество столбцов"))
        Dim a(n, m) As Integer
        Dim b(n) As Integer
        For i = 0 To n - 1
            For j = 0 To m - 1
                a(i, j) = Val(InputBox("Введите элемент" + Str(i + 1) + "-й строки " + Str(j + 1) + "-го столбца"))
            Next
        Next
        For i = 0 To n - 1
            For j = 0 To m - 1
                List1.Text = Str(a(i, j))
            Next
        Next
        proc1(a, n, m, b)
        Max = b(0)
        Min = b(0)
        For i = 0 To n - 1
            If b(i) > max Then
                max = b(i)
                fm = i
            End If
            If b(i) < min Then
                min = b(i)
                qm = i
            End If
        Next
        If min = max Then
            List2.Text = "Количество макс и мин равны"
        Else
            proc2(a, n, m, fm, qm, c)
            For i = 0 To n - 1
                For j = 0 To m - 1
                    List2.Text = Str(a(i, j)) + " "
                Next
            Next
        End If
    End Sub
    Private Sub proc1(ByVal a(,) As Integer, ByVal n As Integer, ByVal m As Integer, ByRef b() As Integer)
        Dim i, j, flag As Integer
        flag = 0
        For i = 0 To n - 1
            For j = 0 To m - 1
                If a(i, j) Mod 2 = 1 Then flag = flag + 1
            Next
            b(i) = flag   !!!!<--- ОШИБКА В ЭТОЙ СТРОКЕ!!!!
            flag = 0
        Next
    End Sub
    Private Sub proc2(ByVal a(,) As Integer, ByRef n As Integer, ByVal m As Integer, ByVal fm As Integer, ByVal qm As Integer, ByRef c(,) As Integer)
        Dim i, j As Integer
        For i = 0 To n - 1
            For j = 0 To m - 1
                c(i, j) = a(i, j)
            Next
        Next
 
        For j = 0 To m - 1
            c(fm, j) = a(qm, j)
            c(qm, j) = a(fm, j)
        Next
    End Sub

Dim b() = {12, 5, 16, 12, 25, 6}
Dim Max As Integer = b.Max
Dim Min As Integer = b.Min
Dim iMax As Integer = Array.IndexOf(b, Max)
Dim iMin As Integer = Array.IndexOf(b, Min)

Dim a(n, m) As Integer
Dim b(n) As Integer

For i = 0 To n - 1
    For j = 0 To m - 1
        '…
    Next
Next