Describe the bug

Providing NULL to the send callback of ucp_put_nb leads to a seg. fault.

From looking at the code with @shamisp the problem appears to be that ucp_request_set_callback unconditionally sets UCP_REQUEST_FLAG_CALLBACK.

Thanks four your helpful application but I have small suggestion.
I would like to see examples in sentences like

essen:
    example: Olga isst einen Apfel.
    example translation: Olga eats an apple.

Examples may be more helpful if the verb changes randomly. Another interaction with the essen may reveal participle example of verb instead of 3rd person which shown up previou

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