TreeView Control - Object reference not set to an instance of an object.

I put 3 treeview controls on my master page, and binded them
to 3
XmlDataSource's. I keep getting the below error message,
but I am
having trouble figuring out what the problem is?

Exception Details: System.NullReferenceException: Object
reference not
set to an instance of an object.


Source Error:


An unhandled exception was generated during the execution of
the
current web request. Information regarding the origin and
location of
the exception can be identified using the exception stack
trace below.


Stack Trace:


[NullReferenceException: Object reference not set to an
instance of an
object.]
   System.Xml.XmlLoader.Load(XmlDocument doc, XmlReader
reader, Boolean

preserveWhitespace) +39
   System.Xml.XmlDocument.Load(XmlReader reader) +159


System.Web.UI.WebControls.XmlDataSource.PopulateXmlDocument(
XmlDocument

document, CacheDependency& dataCacheDependency,
CacheDependency&
transformCacheDependency) +388
   System.Web.UI.WebControls.XmlDataSource.GetXmlDocument()
+204
  
System.Web.UI.WebControls.XmlHierarchicalDataSourceView.Sele
ct() +42

   System.Web.UI.WebControls.TreeView.DataBindNode(TreeNode
node) +479
   System.Web.UI.WebControls.TreeView.PerformDataBinding()
+136


System.Web.UI.WebControls.HierarchicalDataBoundControl.Perfo
rmSelect()
+93
   System.Web.UI.WebControls.BaseDataBoundControl.DataBind()
+99
   System.Web.UI.WebControls.TreeView.DataBind() +24
  
System.Web.UI.WebControls.BaseDataBoundControl.EnsureDataBou
nd() +92

  
System.Web.UI.WebControls.BaseDataBoundControl.OnPreRender(E
ventArgs

e) +33
   System.Web.UI.WebControls.TreeView.OnPreRender(EventArgs
e) +183
   System.Web.UI.Control.PreRenderRecursiveInternal() +148
   System.Web.UI.Control.PreRenderRecursiveInternal() +233
   System.Web.UI.Control.PreRenderRecursiveInternal() +233
   System.Web.UI.Control.PreRenderRecursiveInternal() +233
   System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean
includeStagesAfterAsyncPoint)
+4440


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