There are quite a few place where you could get this error. Since you have not specified where, let me point out the most probable place.
If error is at this line:
Dim sqltext As String = "Update Budget Set BudgetYear =" & budgetYearTextBox.Text & ", YearBudgetAmount =" & budgetAmountTextBox.Text & " where AgentID =" & idlbl.Text & ""
Reason might be the FindControls in the above 3 lines did not return any of the control (Label or Textbox) and thus when you try to access it value, it breaks! Make sure they are not null and proper controls.
Another place could while you use connection string. Do look it.
BTW, a simple use of DEBUGGER should tell the line and issue easily. Try it.