Version: FreeRTOS-Plus-FAT-160919a-MIT In fffile.c: FFClose() @ line 2970:

ffconfigFREE( pxFile );

if( FF_isERR( xError ) == pdFALSE )
{
    xError = FF_FlushCache( pxFile->pxIOManager ); /* Ensure all modified blocks are flushed to disk! */
}

Fix: move ffconfigFREE(…) after the if block.