Adding Multiple TreeNode Levels with Multi threading - vb.net

Question

i'm having alot of trouble trying to get a background worker properly functioning to populate a couple of TreeView Nodes. In said TreeView i have multiple levels of Nodes. For example
FileName
Model
Test & Result.
Model refers to a MicroStation Model (CAD Drawing Program), easiest way to explain it is a Sheet within a Spreadsheet.
I'm using a FileDialog to select files, once the files are selected each filename is added to the TreeView with its own Node.
The idea is that the program will then open each file, scan each model and add a sub TreeNode under the files Node wih the type of test and the result.
The DoWork function for the Background worker is below. I have removed alot of the code to simply my post. But there are 7 "tests" that the program does, i have included 2.
In the below example, "CheckFonts" is a function that just counts the text elements in a file and returns a number.
Private Sub BackgroundWorker1_DoWork(sender As Object, e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork
For i As Integer = 0 To m_CountTo
If BackgroundWorker1.CancellationPending Then
e.Cancel = True ' Set Cancel to True
Exit For
End If
Dim sError As String = ""
Dim bBorderFound As Boolean = False
Dim oDesignFile As DesignFile
Dim oModel As ModelReference
Dim oFontNode As New TreeNode
Dim oFontResultNode As New TreeNode
For Each oNode As TreeNode In trvItems.Nodes
Dim ustn As New MicroStationDGN.Application
oDesignFile = ustn.OpenDesignFileForProgram(oNode.Text, True)
For Each oModel In oDesignFile.Models
'####### Checks for Items on Default Level #######
If bDefaultPass = True Then
Dim iDefaultItems As Long
iDefaultItems = DefaultItems(oModel)
If iDefaultItems > 0 Then
sDefaultMessage = "There are " & iDefaultItems & " items on the Default Level"
bDefaultPass = False
Else
sDefaultMessage = "There are no items on the Default Level"
bDefaultPass = True
End If
End If
'####### Checks for Non Standard Fonts #######
If bFontPass = True Then
Dim iFontCheck As Long
iFontCheck = CheckFonts(oModel)
If iFontCheck > 0 Then
sFontMessage = "There are " & iFontCheck & " Text Elements that use a Non Standard Font."
bFontPass = False
ElseIf iFontCheck = -99999 Then
sFontMessage = "There are some corrupt or invalid Fonts used in the Design File"
bFontPass = False
Else
sFontMessage = "All Text Elements use the Correct Font"
bFontPass = True
End If
End If
Next ' End Model
oFontNode = oNode.Nodes.Add("Font Check")
oFontResultNode = oFontNode.Nodes.Add("")
If bFontPass = True Then
oFontResultNode.Text = "PASS - " & sFontMessage
oFontResultNode.ImageIndex = 0
oNode.Collapse()
Else
oFontResultNode.Text = "FAIL - " & sFontMessage
bPass = False
oFontResultNode.ImageIndex = 1
oFontNode.ImageIndex = 1
oNode.Expand()
oFontNode.Expand()
End If
oDefaultItemsNode = oNode.Nodes.Add("Default Items Check")
oDefaultItemsResultNode = oDefaultItemsNode.Nodes.Add("")
If bDefaultPass = True Then
oDefaultItemsResultNode.Text = "PASS - " & sDefaultMessage
oDefaultItemsResultNode.ImageIndex = 0
oNode.Collapse()
Else
oDefaultItemsResultNode.Text = "FAIL - " & sDefaultMessage
oDefaultItemsResultNode.ImageIndex = 1
oDefaultItemsResultNode.ImageIndex = 1
oNode.Expand()
bPass = False
End If
Next ' End File
Next
End Sub
I have worked with Background Workers before but this is a bit more complex to what i have done, i understand that you cant update controls from a different thread and that Invoke is used to pass it information. But i'm confused how to do it with multiple nodes. The best example I saw was here
Adding nodes to treeview with Begin Invoke / Invoke
But with multiple nodes the code became quite confusing and didn't work. The error message keeps coming up regarding the Invoke / BeginInvoke being called.
So, i guess my main question is where i would call the Invoke command to best make use of the Background worker?
Thanks in advance!!