how do i get a com addin and .net form to talk to each other?

I've created a COM addin for several office applications (word,excel,ppt) and have put a commandbarbutton into each. Clicking this button calls a form. This form acts as a user interface to the current documents document properties so that the user only need click onto menuitems in several context menu boxes, selected a couple of keywords and they are set as properties (builtin and custom) for that document. The only problem is that I keep getting an object reference not set. I just don't know how to pass the document type detected in the com addin e.g.
if typeof (application) is word.application then
dim docType as word.document
wordApp = CType(application, word.application)
excelApp = nothing
pptApp = nothing
else if ...

to the form! The form uses oCustomProperties = docType.BuiltInDocumentProperties
etc to get anything usefule done so the addin needs to pass the docType to the form ByVal. I just don't know how to do this. Please help!

Eternally indebted,

neuralSea