USBX: hard fault after reading fx_directory_information_get

Hi team,

I'm using stm32f746disco and trying to implement USBx as a host with AZURE RTOS. Everything goes well until I want to read the file size using the "fx_directory_information_get" function. The function runs without failure and returns the values correctly, but if I unplug the USB, the MCU goes to a hard fault. 

Questions:

1) Am I using the right function to read the file size?

2) Does it need to call any other function before or after "fx_directory_information_get" somehow to close or flush the media?