add missing nullptr check

See
http://crashreport.libreoffice.org/stats/crash_details/173c2c82-7241-44b4-8394-2b5635f7f938

Change-Id: I2a03c6d6a77e3c872b8507c72b47a05db20fd564
Reviewed-on: https://gerrit.libreoffice.org/27791Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>

svx/source/dialog/srchdlg.cxx

@@ -2377,8 +2377,12 @@ SfxChildWinInfo SvxSearchDialogWrapper::GetInfo() const
 
 static vcl::Window* lcl_GetSearchLabelWindow()
 {
+    SfxViewFrame* pViewFrame = SfxViewFrame::Current();
+    if (!pViewFrame)
+        return nullptr;
+
     css::uno::Reference< css::beans::XPropertySet > xPropSet(
-            SfxViewFrame::Current()->GetFrame().GetFrameInterface(), css::uno::UNO_QUERY_THROW);
+            pViewFrame->GetFrame().GetFrameInterface(), css::uno::UNO_QUERY_THROW);
     css::uno::Reference< css::frame::XLayoutManager > xLayoutManager;
     xPropSet->getPropertyValue("LayoutManager") >>= xLayoutManager;
     css::uno::Reference< css::ui::XUIElement > xUIElement =