potential null pointer access / _initializerCallback not initialized

[ddmesh]

ArduinoCore-mbed/libraries/Ethernet/src/Ethernet.cpp

Line 8 in dd5abb6

_initializerCallback();

When user creates his own object and passes a null pointer for EthernetInterface()
arduino::EthernetClass Ethernet(nullptr); then the uninitialized function pointer _initializerCallback is called accessing some random memory location.

Also as the comment asks "What is this function for?". If this is not defined or documentated, then you should remove it completely to avoid such errors. Beside of this, this "callback" functions makes no sence, as this is called from EthernetClass::begin() synchronously. So the user can call any function afterwards. It is bad design to move private project specific code into public libraries.