modernize C++: use nullptr

src/Gui/Tree.h

@@ -60,7 +60,7 @@ public:
     explicit TreeWidget(const char *name, QWidget* parent=nullptr);
     ~TreeWidget() override;
 
-    static void setupResizableColumn(TreeWidget *tree=0);
+    static void setupResizableColumn(TreeWidget *tree=nullptr);
     static void scrollItemToTop();
     void selectAllInstances(const ViewProviderDocumentObject &vpd);
     void selectLinkedObject(App::DocumentObject *linked);

src/Gui/UiLoader.h

@@ -128,7 +128,7 @@ public:
      *
      * @see https://github.com/FreeCAD/FreeCAD/issues/8708
      */
-    static std::unique_ptr<UiLoader> newInstance(QObject *parent=0);
+    static std::unique_ptr<UiLoader> newInstance(QObject *parent=nullptr);
 
     ~UiLoader() override;
 

src/Gui/ViewProviderLink.cpp

@@ -3165,7 +3165,7 @@ std::map<std::string, App::Color> ViewProviderLink::getElementColors(const char
     }
     std::map<std::string, App::Color> ret;
     for(const auto &v : colors) {
-        const char *pos = 0;
+        const char *pos = nullptr;
         auto sobj = getObject()->resolve(v.first.c_str(),nullptr,nullptr,&pos);
         if(!sobj || !pos)
             continue;