Runtime error: Startup ODCL app + (command "_.open" ...)

I have created an OpenDCL based app that is automatically started whenever a drawing is opened. Things work fine but whenever I use (command "_.open fileName) I get this runtime error:

Error: NIL value not allowed.

Function: dcl-Form-Show

Using:
BricsCAD 16
OpenDCL 8.1.3.1

To test this you can add:

(load "C:\\Program Files (x86)\\OpenDCL Studio\\ENU\\Samples\\@AllSamples.lsp")

to your *doc*.lsp (on_doc_load.lsp if you are a BricsCAD user).
And then (after restarting the CAD-program of course) copy-paste this in the Command Bar:

(command "_.open fileName)

Where fileName must be a valid file name.

My workaround is to use (dcl-SendString) instead of (command).

The application is available here (freeware, for BricsCAD only):
http://www.b-k-g.nl/bkg_browser.html

I have sent in a Support Request to the Bricsys team regarding this issue.

Is it solved?

For me seems that Bricscad is awaiting a variable for "dcl-form-show". It should be optional, but I now always use

(setq result (dcl-form-show mydialog))

Maybe that's the problem?

@Peter2:
Thank you for your answer. When I wrote my SR (SR76961) I was still using BricsCAD V16. According to the response from Bricsys some Lisp related timing issues were fixed in V17. But this has not solved this problem as a test with V18 demonstrates. Assigning the return value from dcl-form-show to a variable also does not fix it.

BricsCad 18.2.04
OpenDCL 8.2.0.1 (the installer has a different version number: OpenDCL.Studio.ENU.8.2.2.1.msi).