0x00007ffd977c04a8 (qt5sqld.dll) (in a.exe): 0xc0000005: an access violation occurred when reading position 0x0000000000000010

There is a problem when modifying other people's code today ：
I want to use

QSqlDatabase::addDatabase("QMYSQL");

however , It can't go anywhere .
I debug , Want to find out the cause of the problem ：

result , I find , I haven't allocated space for the current function when I call it , But after calling the function , Just assigned .

The initialized function was not considered when adding this variable .
After modifying the order of calling functions and allocating space , Re commissioning ：