Radio buttons fire in VB net at form load - vb.net

Question

I have a simple VB net form with two radio buttons in a group box and button outside the group box that calls another form that sets up other parameters. The radio buttons both send data over the serial port.
Form1_load event has a boolean value form_loading = True. This is checked in the rbtn handler and if true should exit the subroutine. On debug, the check changed even fires one button's event that is checked at design time and at this point the form_loading value is set to false and I have no idea why. There is no form_loading = false statement. If I remove the rbtn handler, the form_loading = True persists when the other form is called and returned. The groupbox with buttons is activated as it sees a rbtn1-CheckChanged when the form loads and form_loading value get set to false. I suspect that the rbtn event is firing as the form begins to load, before the form_loading = True statement is reached, but how do I stop it firing the button event?
As its stands, when debug start, there is an IO exception error:
Serialport is closed, and the code associated with the button is in
the buffer to send to a (closed) com port
Private Sub rbtnDon_CheckedChanged(sender As Object, e As EventArgs) Handles rbtnDon.CheckedChanged
If form_loading Then
Exit Sub
ElseIf rbtnDoff.Checked = True Then 'event fires when other button checkchanged = true, this stops it
Exit Sub
Else
data_out = (SOT + "N" + EOT)
SendtoBoard(data_out)
End If
End Sub`enter code here`

Answer 1

Thanks for your help Everyone. Elsewhere I found this tip: Remove the "handles rbtnDon checkchanged" from the sub:
Private Sub rbtnDon_CheckedChanged(sender As Object, e As EventArgs)handles rbtnDon checkchanged
and add this to the form1_load sub:
AddHandler rbtnDon.CheckedChanged, New EventHandler(AddressOf rbtnDon_CheckedChanged)
That seems to have solved the problem completely.
I could not add the startup code on my original post as for some reason the box will not accept two lots of separate code - or I am not doing it right:
It is here:
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
For i As Integer = 0 To My.Computer.Ports.SerialPortNames.Count - 1
ccbComPort.Items.Add(My.Computer.Ports.SerialPortNames(i))
Next
If ccbComPort.Items Is Nothing Then
lblMessage.ForeColor = Color.Red
lblMessage.Text = "No Serial Ports found"
Else
ccbComPort.Text = My.Settings.oldPort
End If
ccbBaudrate.Text = My.Settings.oldBaud
pnlComPorts.Visible = True
'form_loading = True
rbtnDon.Enabled = True
AddHandler rbtnDon.CheckedChanged, New EventHandler(AddressOf rbtnDon_CheckedChanged)
End Sub
Originally I had one form with a panel for the Com port setup and everything was fine, the issue only started when I moved the port set up to form2, after which the rbtnDon event fired before form1 started.
I did try the Sub New() approach but then I just get a small blank form on debug. Adding Form1_load to it results in "NotImplemented Exception".
The method described above seems OK and fairly simple to add but maybe it is not good practice?