Fixed crash on "Edit block" function.

I simply added a check for nullptr and seems it is enough.

Index: librecad/src/ui/dock_widgets/views_list/lc_namedviewslistwidget.cpp
==================================================================
--- librecad/src/ui/dock_widgets/views_list/lc_namedviewslistwidget.cpp
+++ librecad/src/ui/dock_widgets/views_list/lc_namedviewslistwidget.cpp
@@ -129,14 +129,16 @@
 
 void LC_NamedViewsListWidget::setGraphicView(RS_GraphicView* gv, QMdiSubWindow* w) {
     LC_ViewList* viewsList = nullptr;    
     if (gv != nullptr) {
         RS_Graphic *graphic = gv->getGraphic();
-        linearFormat = graphic->getLinearFormat();
-        precision = graphic->getLinearPrecision();
-        drawingUnit = graphic->getUnit();
-        viewsList = graphic->getViewList();
+        if (graphic != nullptr) {
+            linearFormat = graphic->getLinearFormat();
+            precision = graphic->getLinearPrecision();
+            drawingUnit = graphic->getUnit();
+            viewsList = graphic->getViewList();
+        }
     }
     graphicView = gv;
     window = w;
     setViewsList(viewsList);
 }