Revert "cast to wrong type, can cause crash when opening Options-Writer-AutoCa

Wrong fix: GetParent can return pointer to SwCaptionOptDlg which is
derived from SfxSingleTabDialog.
This reverts commit a5546942.

sw/source/ui/config/optload.cxx

@@ -769,6 +769,11 @@ void SwCaptionOptPage::SaveEntry(SvLBoxEntry* pEntry)
 IMPL_LINK_NOARG(SwCaptionOptPage, ModifyHdl)
 {
     String sFldTypeName = aCategoryBox.GetText();
+
+    SfxSingleTabDialog *pDlg = (SfxSingleTabDialog *)GetParent();
+    PushButton *pBtn = pDlg->GetOKButton();
+    if (pBtn)
+        pBtn->Enable(sFldTypeName.Len() != 0);
     sal_Bool bEnable = aCategoryBox.IsEnabled() && sFldTypeName != sNone;
 
     aFormatText.Enable(bEnable);