Cleanup: Add static assert for tree-view getter template

These kind of static asserts for the base type of a template parameter
are useful, and can avoid wrong API usage.

source/blender/editors/include/UI_tree_view.hh

@@ -455,6 +455,8 @@ inline ItemT &TreeViewItemContainer::add_tree_item(Args &&...args)
 
 template<class TreeViewType> TreeViewType &AbstractTreeViewItemDropController::tree_view() const
 {
+  static_assert(std::is_base_of<AbstractTreeView, TreeViewType>::value,
+                "Type must derive from and implement the AbstractTreeView interface");
   return static_cast<TreeViewType &>(tree_view_);
 }