Check return value of getMDIViewPage() a second time
Check return value of getMDIViewPage() a second time

When exporting a hidden TechDraw::Page with exportPageAs[Pdf|SVG] a
Segmentation Fault occured.

The return value of getMDIViewPage() was only checked for the first call, but
not for the second call. showMDIViewPage() silently returns, when the
TechDraw::Page is hidden and does not load the page.

Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master,Fat-Zer/FreeCAD_sf_master