[SUTK] Fix for segfault when the last tab is destroyed, added more co…

…mments on onStepAll() method in AnimationGroup

sutk/include/sutk/AnimationEngine.hpp

@@ -56,6 +56,8 @@ namespace SUTK
 			// Called when all the animations in this group have ended
 			virtual void onWhenAllEnd() noexcept { }
 			// Called after when onStep() is invoked over all animations in this group
+			// NOTE: It is called even if the last stepped animation is ended resulting in zero animation count.
+			// So, if you're destroy some objects in onEnd() of an Animation then make sure to check of nulls.
 			virtual void onStepAll() noexcept { }
 		};
 		class AnimContextBase : public UIDriverObject

sutk/source/NotebookView.cpp

@@ -358,7 +358,10 @@ namespace SUTK
 	void TabAnimGroup::onStepAll() noexcept
 	{
 		auto* tabBar = m_notebook->getTabBar();
-		tabBar->scrollToTab(tabBar->getSelectedTab());
+		// onStepAll() is called even for the last (only one is left) stepped animation and that animation might destroy the only tab left in the TabBar
+		// Causing tabBar->getSelectedTab() to return nullptr, so we need to check for null here.
+		if(tabBar->getSelectedTab())
+			tabBar->scrollToTab(tabBar->getSelectedTab());
 	}
 	TabAnimGroup::TabAnimGroup(UIDriver& driver, NotebookView* notebook) noexcept : AnimGroup(driver), m_notebook(notebook)
 	{