OnChecked Textbox Empty Value Issue

Hello,

I'm in the learning process and I got  into a problem! I want to do a SUM from two TextBoxes and a drop down menu that trigers when I checked a CheckBox, and that works just fine!

QTextBox - default value:Empty - UTextBox - Default value: 0.25 TDropDown - Default value: 5 to 15

But!  if  QTextBox is empty it returns and "error message"! How can I validate ATextBox that "IF" it has value will do Result.ToString and "If it is empty will do a response.write("Please enter amount.!")

Can one of you guys give  me a hand! :)

Here is the my coding

  Protected Sub CCheckBox_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs)

        Dim Click As CheckBox = CType(FormView1.FindControl("CCheckBox"), CheckBox)        
     & ...