I'm not sure if it wil be useful, but i'll give a go at interpreting the stacktrace.
What i can make of following sections of the stacktrace:
at MyFirstModule.Assistant (JavaAction : 'Assistant - Send message')
Caused by: java.lang.NullPointerException: null
Is that you probably have a java action called “Assisant”, which calls java method AssisentService.sendMessage(). The sendMessage causes a nullpointer exception. The AssisentService is probably part of a jar file that you put in the userlib folder of your Mendix project.
Nullpointer exceptions normally mean that the code expects a variable or constant to contain a value, but it is empty instead. Could it be you are missing a parameter, or passing empty as a value in a parameter? May some debugging or extra loglines could create extra insight?
Would the library need some sort of extra configuration in the form of a properties-files or constants of some sort?
I hope this helps a little.