Hmmm, lots of beer and confusion (on my part). In your case, I don't think the combo can be null after the update.
Anyway, I thought there was only one report requiring start and end dates. FWIW, here's another take on how it might go (warning: air code to follow).
Code:
Private Sub cboReports_AfterUpdate()
If Me.cboReports = "Personnel Arrived (Date Range)" Then
If IsNull([StartDate]) OR IsNull([EndDate)]) Then
Msgbox "A start and end date is required for the selected report."
Exit Sub
End if
If Not IsDate(Me.txtStartDate) OR Not IsDate(Me.txtEndDate) Then
Msgbox "Only valid dates can be entered into the Begin and Start date fields."
Exit Sub
End If
End If
End Sub
What my air code would not do (assuming it's written correctly) is ensure the end date is not before the start date. You might be surprised that I would consider that, but hey, there's always someone who will surprise you. Nor will it validate that there are valid dates or no Nulls for the other reports, but I provide it as an example of how one might want to validate date parameters.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.