Comment 1 for bug 1756915

qemuDomainAttachDeviceLiveAndConfig
  if (virDomainDefCompatibleDevice(vm->def, dev_copy, NULL) < 0)

  -> virDomainDefCompatibleDevice
     # oldDev is the NULL above
     .oldInfo = virDomainDeviceGetInfo(oldDev),

     -> virDomainDeviceGetInfo
        # device is the NULL pointer
        switch ((virDomainDeviceType) device->type) {

This very likely is either fixed already or an effect of the stable fixes lacking a dependent change.

Checking upstream vs stable branch ...