Implement initial fix to #25

Appears to correctly address the problem leading to RTE 91 when
the header option was enabled and an entire row/column was
selected that did not intersect .UsedRange

Closes #25.

Author: bskinn

src/UFExporter.frm

@@ -442,8 +442,12 @@ Private Function getHeaderRangeAddress() As String
     '
     ' Or, if header export is deselected, report accordingly
 
+    Dim headerRange As Range
+    
+    Set headerRange = getHeaderRange
+    
     If ChBxHeaderRows.Value Then
-        If checkHeaderRowValues Then
+        If Not headerRange Is Nothing And checkHeaderRowValues Then
             getHeaderRangeAddress = getHeaderRange.Address( _
                         RowAbsolute:=False, ColumnAbsolute:=False _
             )