Why did QEMU give me the (null) instead of a string?

Question

I am learning OS recently, but I got a problem that QEMU outputs (null) instead of a string what I want when I was debugging the program.

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The QEMU emulator version is 4.2.1 (Debian 1:4.2-3ubuntu6.18), and I use it in WSL(Windows Subsystem for Linux) that don't have GUI. I run the QEMU by command qemu-system-i386 -hda myprogram.img -nographic. After the commmand, what shows in QEMU interface is memory management: (null), but it should be memory management:default_pmm_manager. Then I debugged my program in QEMU for Ubuntu that has GUI and its version is 2.0.0. I really have the output memory management:default_pmm_manager . So it dues to QEMU vesion?
Here are some functions code in my program.

struct pmm_manager {
    const char *name;                                 // XXX_pmm_manager's name                      
    void (*init)(void);                               // initialize internal description&management data structure
                                                      // (free block list, number of free block) of XXX_pmm_manager 
    void (*init_memmap)(struct Page *base, size_t n); // setup description&management data structcure according to
                                                      // the initial free physical memory space 
    struct Page *(*alloc_pages)(size_t n);            // allocate >=n pages, depend on the allocation algorithm 
    void (*free_pages)(struct Page *base, size_t n);  // free >=n pages with "base" addr of Page descriptor structures(memlayout.h)
    size_t (*nr_free_pages)(void);                    // return the number of free pages 
    void (*check)(void);                              // check the correctness of XXX_pmm_manager 
};

const struct pmm_manager *pmm_manager;

const struct pmm_manager default_pmm_manager = {
    .name = "default_pmm_manager",
    .init = default_init,
    .init_memmap = default_init_memmap,
    .alloc_pages = default_alloc_pages,
    .free_pages = default_free_pages,
    .nr_free_pages = default_nr_free_pages,
    .check = default_check,
};

static void
init_pmm_manager(void) {
    pmm_manager = &default_pmm_manager;
    cprintf("memory management: %s\n", pmm_manager->name);
    pmm_manager->init();
}

I really want to have the right output in QEMU of WSL. Is there any solutions?