How can I handle this error: 'object cannot be cast from dbnull to other types.'?

To add to what Dave says, it's even worse - you open a connection to the DB which can never be used as it is only stored in a local variable, which goes out of scope at the end of the Form.Load event handler.

Since it can't be accessed at all outside the method (in fact all local variables no longer exists when the method ends) the connection you create is completely useless, and just takes up scarce resources on the server.

You should always us a Using block around the creation of any DB objects (Connection, Command, Reader, and Adapter) to automatically close and Dispose them when the block ends:

Using con As New SqlConnection(strConnect)
	con.Open()
	Using cmd As New SqlCommand("SELECT iD, description FROM myTable", con)
		Using reader As SqlDataReader = cmd.ExecuteReader()
			While reader.Read()
				Dim id__1 As Integer = CInt(reader("iD"))
				Dim desc As String = DirectCast(reader("description"), String)
				Console.WriteLine("ID: {0}" & vbLf & "    {1}", iD, desc)
			End While
		End Using
	End Using
End Using