Fix T93250: Crash in spreadsheet tree view after recent commit

It seems that update_from_old assumed there would be an old
tree view available in the old block. This works for the asset browser
because the tree is always drawn, but for the spreadsheet that depends
on having an active object, which isn't necessarily always true.

Differential Revision: https://developer.blender.org/D13301

source/blender/editors/interface/tree_view.cc

@@ -111,7 +111,10 @@ void AbstractTreeView::update_from_old(uiBlock &new_block)
 
   uiTreeViewHandle *old_view_handle = ui_block_view_find_matching_in_old_block(
       &new_block, reinterpret_cast<uiTreeViewHandle *>(this));
-  BLI_assert(old_view_handle);
+  if (old_view_handle == nullptr) {
+    is_reconstructed_ = true;
+    return;
+  }
 
   AbstractTreeView &old_view = reinterpret_cast<AbstractTreeView &>(*old_view_handle);