Fix replacement of event queue background. This was discovered when EventQueue::background was called with a nullptr. It crashes the software as it tries to call into a nullptr.
Fix replacement of event queue background.
This was discovered when EventQueue::background was called with a nullptr.
It crashes the software as it tries to call into a nullptr.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed