Place Equipment From Ribbon Revit 2015: Transaction already started - vb.net

Question

Stack,
I'm programming an external application that works in Autodesk's Revit (2015). I've created a button on the ribbon that allows the user to place a special equipment that I'll use later to read and pass on to another process. I using 'TheBuildingCoder's' 'Family API' examples to do so. The button on the ribbon has an external command set in the .addin file in the ProgramData Directory...
<AddIn Type="Command">
<Assembly>C:\GSN Programs\MyDll.dll</Assembly>
<AddInId>{97715E4F-EA48-4690-8C62-B5D4836FF452}</AddInId>
<FullClassName>RcarsPlugIn.PlaceEquipment</FullClassName>
<VendorId>MyCompany, LLC</VendorId>
<Text>Place Equipment</Text>
<VisibilityMode>AlwaysVisible</VisibilityMode>
<Discipline>Any</Discipline>
<LanguageType>Unknown</LanguageType>
</AddIn>
On the button press, I put the command data into a global variable to use throughout the program...
If IsNothing(gv_oGo) Then
gv_oGo = New clsGeneralOperations
gv_oGo.CachedCommandData = exCommandData
gv_oGo.UiApp = exCommandData.Application.ActiveUIDocument.Application
End If
With the CommandData cached I move to place the equipment by user pick...
uiDoc = gv_oGo.UiApp.ActiveUIDocument
oSym = oRF.FindElement(doc, GetType(FamilySymbol), "MyEQUIP")
uiDoc.PromptForFamilyInstancePlacement(oSym)
Public Function FindElement(doc As Document, targetType As Type, sTargetName As String) As Element
Return New FilteredElementCollector(doc).OfClass(targetType).FirstOrDefault(Function(e) e.Name.Equals(sTargetName))
End Function
That's where the problem comes into play. I get an error message sent back from Revit stating that "Placement is not permitted in an already modifiable document. The active transaction must be closed first." The problem is, I haven't started a different transaction. The button onthe ribbon is the first that touched when Revit starts.
Is there a way to loop through the open transactions and find one that is open? Is the document in some state that I don't understand? I'm not sure which direction to turn here... any help would be appreciated.
Thanks,
Runnin...

Answer 1

Ok, one clarifying question - the exception is occurring on
uiDoc.PromptForFamilyInstancePlacement(oSym) line correct? if so, try using a subtransaction to complete your command instead:
Document doc = uiDoc.Document;
using (SubTransaction subtr_fam = new SubTransaction(doc))
{
try
{
uiDoc.PromptForFamilyInstancePlacement(oSym);
}
catch(Exception e)
{
Console.WriteLine(e.StackTrace.ToString());
}
}