[TD]fix potential nullptr while restoring

src/Mod/TechDraw/App/DrawViewDimension.cpp

@@ -219,6 +219,11 @@ App::DocumentObjectExecReturn *DrawViewDimension::execute(void)
 //    Base::Console().Message("DVD::execute() - %s\n", getNameInDocument());
     if (!keepUpdated()) {
         return App::DocumentObject::StdReturn;
+    }
+    DrawViewPart* dvp = getViewPart();
+    if (dvp == nullptr) {
+        Base::Console().Message("DVD::execute - no DVP!\n");
+        return App::DocumentObject::StdReturn;
     }
 
     //any empty Reference2D??
@@ -716,8 +721,11 @@ double DrawViewDimension::getDimValue()
         }
         return result;
     }
+    if  (getViewPart() == nullptr) {
+        return result;
+    }
 
-    if (!getViewPart()->hasGeometry()) {                              //happens when loading saved document
+    if  (!getViewPart()->hasGeometry() ) {                              //happens when loading saved document
         return result;
     }
 

src/Mod/TechDraw/Gui/QGIViewBalloon.cpp

@@ -501,6 +501,9 @@ void QGIViewBalloon::draw_modifier(bool modifier)
     show();
 
     const TechDraw::DrawViewPart *refObj = balloon->getViewPart();
+    if (refObj == nullptr) {
+        return;
+    }
     if(!refObj->hasGeometry()) {                                       //nothing to draw yet (restoring)
         balloonLabel->hide();
         hide();

src/Mod/TechDraw/Gui/QGIViewDimension.cpp

@@ -675,6 +675,9 @@ void QGIViewDimension::draw()
     }
 
     const TechDraw::DrawViewPart *refObj = dim->getViewPart();
+    if (refObj == nullptr) {
+        return;
+    }
     if(!refObj->hasGeometry()) {                                       //nothing to draw yet (restoring)
         datumLabel->hide();
         hide();

src/Mod/TechDraw/Gui/QGIViewPart.cpp

@@ -830,6 +830,9 @@ void QGIViewPart::drawSectionLine(TechDraw::DrawViewSection* viewSection, bool b
     if (!viewPart)  {
         return;
     }
+    if (viewSection == nullptr) {
+        return;
+    }
 
     if (!viewSection->hasGeometry()) {
         return;