This error normally comes when i have left my computer idle for like 20 min please help how I can avoid it 

Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:


Line 3:  
Line 4:      Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Line 5:          LoggedIn.Text = Session("Username").ToString()
Line 6:  
Line 7:      End Sub

thanks