Access Violation after injecting dll to an executable - c++

Question

I have a DLL which is acting like a hooking engine. I am going to inject this library to processes in order to analysis APIs that they are calling.
It will hook functions like MessageBoxA after loading to address space of the target process. The following method parse IAT table and then try to overwrite MessageBoxA address in IAT table with modified API (my own api).
void triggered()
{
PIMAGE_IMPORT_DESCRIPTOR import_descriptor = import_directory_table();
while (import_descriptor->Name != NULL)
{
m_lib_name = reinterpret_cast<LPCSTR>(import_descriptor->Name + (DWORD_PTR)va_image_base());
m_library = LoadLibraryA(m_lib_name);
if (m_library)
{
m_original_first_thunk = reinterpret_cast<PIMAGE_THUNK_DATA>((DWORD_PTR)va_image_base() + import_descriptor->OriginalFirstThunk);
m_first_thunk = reinterpret_cast<PIMAGE_THUNK_DATA>((DWORD_PTR)va_image_base() + import_descriptor->FirstThunk);
while (m_original_first_thunk->u1.AddressOfData != NULL)
{
m_function_name = reinterpret_cast<PIMAGE_IMPORT_BY_NAME>((DWORD_PTR)va_image_base() + pe::m_original_first_thunk->u1.AddressOfData);
// MessageBoxA redirected to _MessageBox which is a modified version of Sleep API
if (std::string(m_function_name->Name).compare("MessageBoxA") == 0)
{
VirtualProtect((LPVOID)(&m_first_thunk->u1.Function), 8, PAGE_READWRITE, &m_old_protect);
m_first_thunk->u1.Function = (DWORD_PTR)hook::statically::ui::_MessageBoxA;
}
++m_original_first_thunk;
++m_first_thunk;
}
}
import_descriptor++;
}
}
But when Dll loaded into the process, target process when call MessageBoxA, it give me access violation in debugging mode of visual studio. In the following, you will see the code _MessageBoxA.
namespace hook
{
namespace statically
{
namespace ui
{
// MessageBox call redirected to this function.
int _MessageBoxA(HWND hWnd, LPCSTR lpText, LPCSTR lpCaption, UINT uType)
{
if (std::string(lpText).compare("test") == 0)
{
return load_time::prototype::ui::OMessageBoxA(NULL, "Hahahahahhaa", "Hahahahaha", MB_OK);
}
else
{
return load_time::prototype::ui::OMessageBoxA(hWnd, lpText, lpCaption, uType);
}
return 0;
}
}
}
}
Where is my mistake? I did research a lot in google but couldn't find out a solution.