In which cases can I get the error:

    ToolsCollectionBase owning PopupMenu root tool is invalid or null.

when removing a tool from it's own ParentCollection with following code:

var parentCollection = this.Tool.ParentCollection;
if (parentCollection != null)
{
    parentCollection.Remove(this.Tool);
}

Creating the tool is done fairly simple:

var popupMenuTool = new PopupMenuTool(dropNode.ItemKey);
popupMenuTool.SharedProps.Caption = dropNode.Text;

I am using a rather undocumented method to insert the Tool in the PopupMenuTool as I am creating a custom RibbonCustomizationDialog. Maybe there's a problem there... ( --> InsertToolForRibbonCustomization). 

The full version of the code is:

// Remove the Tool from the ToolbarsManager and the dropNode from the UltraTree so we
// can insert a PopupMenuTool instead.
var dropNodeIndex = parentRibbonGroupNode.Group.Tools.IndexOf(dropNode);
parentRibbonGroupNode.Group.Tools.Remove(dropToolNode.Tool);
dropNode.Remove();
 
// Create PopupMenuTool from DropNode
var popupMenuTool = new PopupMenuTool(dropNode.ItemKey);
popupMenuTool.SharedProps.Caption = dropNode.Text;
 
// Insert PopupMenuTool in the RibbonGroup at index 0
parentRibbonGroupNode.Group.Tools.InsertToolForRibbonCustomization(
    dropNodeIndex,
    popupMenuTool,
    isCopy,
    excludedToolKeys);
 
// Create UltraTreeNode from PopupMenuTool
var popupMenuNode = this.CreateNodeForTool(popupMenuTool, true, null, true);
if (popupMenuTool.GetIsHiddenByMdiMerge())
{
    ((ToolNode)popupMenuNode).ForceVisible = isCopy;
    popupMenuNode.Visible = true;
    this.refreshMerge = true;
}
 
// Insert the popupMenuNode in the UltraTree
parentRibbonGroupNode.Nodes.Insert(0, popupMenuNode);
 
// Now that the dropNode Tool has been converted to a PopupMenuTool, add the dragNode
// to the PopupMenuTool Node. First insert the Tool, then insert the UltraTreeNode.
 
// Insert the Tool
tool = popupMenuTool.Tools.InsertToolForRibbonCustomization(
    0,
    (dragNode as ToolNode).Tool,
    isCopy,
    excludedToolKeys);
 
// Create an UltraTreeNode from the Tool.
newUltraTreeNode = this.CreateNodeForTool(tool, true, null, true);
if (tool.GetIsHiddenByMdiMerge())
{
    ((ToolNode)newUltraTreeNode).ForceVisible = isCopy;
    newUltraTreeNode.Visible = true;
    this.refreshMerge = true;
}
 
// Insert the UltraTreeNode
popupMenuNode.Nodes.Insert(0, newUltraTreeNode);

What I'm actuall trying to do is that when a ButtonTool(A) is dragged over the middle of another ButtonTool(B), then B gets transformed into a PopupMenuTool and the ButtonTool A gets added to that PopupMenuTool. The ButtonTool B is removed. 

Thx, Lieven Cardoen