Creating a login/register form - vb.net

Question

I'm working on a vb form, which allows a person to create an "account". I store the usernames and passwords in two arrays, and extract the information from them. But when I run the program, it comes up with a problem:
"An unhandled exception of type 'System.ArgumentNullException'
occurred in Microsoft.VisualBasic.dll, Additional information: Value
cannot be null."
where the code for the Button2/Register Button is (To be exact:
For i = 0 To (UBound(Usernames))
Could you help me out and tell me what to do differently/how to approach this situation? Here is the code:
Public Class Form1
Dim Usernames() As String
Dim Passwords() As String
Dim CurrName As String
Dim i As Integer
'Login button is pressed
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim Index As Integer
CurrName = TextBox1.Text
For i = 0 To (UBound(Usernames))
If IfRepetition(Usernames, CurrName, i) = True Then
Index = Array.IndexOf(Usernames, TextBox1.Text)
If TextBox2.Text = Passwords(Index) Then
Form3.Show()
Me.Hide()
End If
Else
MsgBox("The username or password is incorrect", MsgBoxStyle.Critical)
End If
Next
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
CurrName = TextBox1.Text
' *** Error (apparently) happens here ***
For i = 0 To (UBound(Usernames))
If IfRepetition(Usernames, CurrName, i) = True Then
MsgBox("This username already exists!")
Else
ReDim Preserve Usernames(UBound(Usernames) + 1)
Usernames(UBound(Usernames)) = TextBox1.Text
ReDim Preserve Passwords(UBound(Passwords) + 1)
Passwords(UBound(Passwords)) = TextBox2.Text
End If
Next
End Sub
Private Function IfRepetition(ByRef Usernames() As String, CurrName As String, i As Integer) As Boolean
Dim j As Integer
'Checks for repetition of a username in the usernames array
IfRepetition = False
For j = 0 To (UBound(Usernames))
If Usernames(j) = CurrName Then
IfRepetition = True
Exit Function
End If
Next
End Function
End Class

Answer 1

The error you're getting is because UBound throws exception if the array is null.
Also using arrays for that isn't a good way, I recommend you use Dictionary for easier, shorter code.
Dim dic As New Dictionary(Of String, String) 'A new dictionary which handles usernames & passwords
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Login button
If String.IsNullOrWhiteSpace(TextBox1.Text) Then 'Check the entered username, if it's empty show error message and return ( don't continue )
MessageBox.Show("Enter the username")
Return
End If
If String.IsNullOrWhiteSpace(TextBox2.Text) Then 'Check the entered password, if it's empty show error message and return ( don't continue )
MessageBox.Show("Enter the password")
Return
End If
If Not dic.ContainsKey(TextBox1.Text) Then 'Check the entered username, if it doesn't exist show error message and return ( don't continue )
MessageBox.Show("The username is incorrect")
Return
End If
If dic(TextBox1.Text) <> TextBox2.Text Then 'Validate entered username and password, if it's wrong show error message and return ( don't continue )
MessageBox.Show("The password is incorrect")
Return
End If
Form3.Show() 'If none of above error messages appear which means the entered username and password are correct, show Form3 and hide this form
Me.Hide()
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
'create account button
If String.IsNullOrWhiteSpace(TextBox1.Text) Then 'Check the entered username, if it's empty show error message and return ( don't continue )
MessageBox.Show("Enter the username")
Return
End If
If String.IsNullOrWhiteSpace(TextBox2.Text) Then 'Check the entered password, if it's empty show error message and return ( don't continue )
MessageBox.Show("Enter the password")
Return
End If
If dic.ContainsKey(TextBox1.Text) Then 'Check the entered username, if it exists show error message and return ( don't continue )
MessageBox.Show("This username already exists")
Return
End If
dic.Add(TextBox1.Text, TextBox2.Text) 'If none of above error messages which means it's okay to create this account, Add the username & password to the dictionary
End Sub