tdf#95798: CRASH: when finish Function wizard on Report builder

Following http://cgit.freedesktop.org/libreoffice/core/commit/?id=844b7287a4ccd8e3d5e458bb1dc6c52e71322b01

Change-Id: Ia5a2cd39cf3fdf87619f55710a188545830b4088
Reviewed-on: https://gerrit.libreoffice.org/19970Reviewed-by: Lionel Elie Mamane <lionel@mamane.lu>
Tested-by: Jenkins <ci@libreoffice.org>

formula/source/ui/dlg/formula.cxx

@@ -1740,7 +1740,8 @@ void FormulaModalDialog::SetFocusWin(vcl::Window *pWin,const OString& nUniqueId)
 
 bool FormulaModalDialog::PreNotify( NotifyEvent& rNEvt )
 {
-    m_pImpl->PreNotify( rNEvt );
+    if (m_pImpl)
+        m_pImpl->PreNotify( rNEvt );
 
     return ModalDialog::PreNotify(rNEvt);
 }