Excel ActiveX ComboBox enters a 0 if a list item isn't selected

I'm new to this site and somewhat new to Excel VBA and comboboxes.  I've hit a wall as far as finding a solution to this problem.  I've had to adapt code from a few different sites (this site, Contextures, OzGrid, Mr. Excel, etc.) to get this worksheet to perform as I've been requested so far.  I've tried to build a worksheet that has several comboboxes based on data validation lists (as well as 1 dependent list).  I have the comboboxes working as a SelectionChange event.  When the user begins data entry on the sheet, the hope is that he/she can simply Tab or Enter through the comboboxes and cells.  So I have to have the comboboxes appear and display some rows of the list when the cell is selected by keyboard or mouse.  When the user selects another cell that has a combobox the same thing happens.  The problem I've now encountered is that if a user selects a cell with a combobox  and then selects another cell without selecting an entry from the previously selected combobox before doing so, a 0 is entered into the cell and is added to the list the combobox is based on (the sheet was set up to allow new entries into the data validation lists--and that's a feature that is wanted for some column cells).  I apologize for the novel-length question, but I'm just trying to give as much background as possible.  Here is the code I have for the worksheet (this includes all the code because I'm not sure if I have conflicting code or if it's isolated to the combobox code):
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Option Explicit
---------------------------------------------------------
Private Sub Worksheet_Change(ByVal Target As Range)
 If Target.Column = 3 Or Target.Column = 4 Then
            My_Sub_B Target
            Else
            My_Sub_A Target
            End If
End Sub
---------------------------------------------------------
Public Sub My_Sub_A(ByVal Target As Range)
 On Error Resume Next
Dim ws As Worksheet
Dim str As String
Dim i As Integer
Dim rngDV As Range
Dim rng As Range

If Target.Count > 1 Then Exit Sub
Set ws = Worksheets("Lists")
 
If Target.Row > 1 Then
  On Error Resume Next
  Set rngDV = Cells.SpecialCells(xlCellTypeAllValidation)
  On Error GoTo 0
  If rngDV Is Nothing Then Exit Sub
 
  If Intersect(Target, rngDV) Is Nothing Then Exit Sub
   
  str = Target.Validation.Formula1
  str = Right(str, Len(str) - 1)
  On Error Resume Next
  Set rng = ws.Range(str)
  On Error GoTo 0
  If rng Is Nothing Then Exit Sub
 
  If Application.WorksheetFunction _
    .CountIf(rng, Target.Value) Then
    Exit Sub
  Else
    i = ws.Cells(Rows.Count, rng.Column).End(xlUp).Row + 1
    ws.Cells(i, rng.Column).Value = Target.Value
    rng.Sort Key1:=ws.Cells(1, rng.Column), _
      Order1:=xlAscending, Header:=xlNo, _
      OrderCustom:=1, MatchCase:=False, _
      Orientation:=xlTopToBottom
  End If

End If
End Sub
----------------------------------------------------
Public Sub My_Sub_B(ByVal Target As Range)
 
If Target.Column = 3 Then
  On Error Resume Next
If Target.Validation.Type = 3 Then
Application.EnableEvents = False
Target.Offset(0, 1).ClearContents
 
End If
End If

exitHandler:
  Application.EnableEvents = True
  Exit Sub

End Sub
-----------------------------------------------
Private Sub TempCombo_KeyDown(ByVal _
        KeyCode As MSForms.ReturnInteger, _
        ByVal Shift As Integer)

'move to next cell on Enter and Tab
Dim varVal As Variant
On Error Resume Next

varVal = --ActiveCell.Value
If IsEmpty(varVal) Then
  varVal = ActiveCell.Value
End If

Select Case KeyCode
  Case 9  'tab
    ActiveCell.Value = varVal
    ActiveCell.Offset(0, 1).Activate
  Case 13 'enter
    ActiveCell.Value = varVal
    ActiveCell.Offset(1, 0).Activate
  Case Else
    'do nothing
End Select

End Sub
------------------------------------------------------------
Private Sub TempCombo_LostFocus()
  With Me.TempCombo
    .Top = 10
    .Left = 10
    .Width = 0
    .ListFillRange = ""
    .LinkedCell = ""
    .Visible = False
    .Value = ""
  End With
End Sub
----------------------------------------------------------------
Private Sub Worksheet_SelectionChange(ByVal Target As Range)
Dim str As String
Dim cboTemp As OLEObject
Dim ws As Worksheet
Dim wsList As Worksheet
Dim lSplit As Long
Set ws = ActiveSheet
Set wsList = Sheets("Lists")
Set cboTemp = ws.OLEObjects("TempCombo")
 
If Target.Count > 1 Then GoTo errHandler
 
  On Error Resume Next
    With cboTemp
    .Visible = False
   End With

On Error GoTo errHandler
If Target.Validation.Type = 3 Then
Application.EnableEvents = False
str = Target.Validation.Formula1
str = Right(str, Len(str) - 1)
       
If Left(str, 4) = "INDI" Then
lSplit = InStr(1, str, "(")
str = Right(str, Len(str) - lSplit)
str = Left(str, Len(str) - 1)
str = Range(str).Value
End If
   
With cboTemp
Visible = True
.Left = Target.Left
.Top = Target.Top
.Width = Target.Width + 15
.Height = Target.Height + 5
.ListFillRange = str
.LinkedCell = Target.Address

End With
cboTemp.Activate
Me.TempCombo.DropDown               'show combobox automatically
End If
 
errHandler:
 Application.ScreenUpdating = True
 Application.EnableEvents = True
 
End Sub