problem with multiselect treeview nodes

hi,

i am a new to c#, i hav 4months experience in C#,
I am populating a treeview in c# from XmlFile, and i have implemented
a multiselection in that treeview by overriding treeview select
events when ctrl or shift is pressed, which was written a seperate
DLL, and added with my project,

now i hav a problem that when i select treeview nodes by pressing
shift key at a same time initially it is creating an exception "Object
reference not set to an reference object" and the application closes.

i know this is bcoz, i have enqued selected nodes to a arraylist. once
it presses shift its starting enqueing, and at that time selected node
is null, thats y it is generating this exception i guess,

can anybody tell me how could i solve this?