How to edit existing expression?

Hello

Below is a journal that creates an expression and if run the second time it should edit expression value that was created in the first run. Do i need another block of code to handle errors?

Imports System
Imports NXOpen

Module create expression

Public theSession As Session = Session.GetSession()
Public workPart As Part = theSession.Parts.Work

Sub Main()

Dim leftPart As String = "12"
Dim expression1 As NXOpen.Expression = Nothing
Dim thr_exp As NXOpen.Expression = Nothing
Dim expression_a As String = "thr_txt"

'create expression

Try
Dim theExp As Expression = workPart.Expressions.FindObject(expression_a)

workPart.Expressions.Edit(theExp, "M""+stringValue( " & leftPart & " )")

Catch ex As NXException
ex.AssertErrorCode(3520016)

expression1 = workPart.Expressions.CreateExpression("String", expression_a & "=""M""+stringValue( " & leftPart & " )")
End Try

End Sub

Private Sub Echo(ByVal output As String)
theSession.ListingWindow.Open()
theSession.ListingWindow.WriteLine(output)
theSession.LogFile.WriteLine(output)
End Sub

Public Function GetUnloadOption(ByVal arg As String) As Integer
Return System.Convert.ToInt32(Session.LibraryUnloadOption.Immediately)
End Function

End Module