Internals: Fixed ImFileOpen not working before context is created.

Fixes #7314

docs/CHANGELOG.txt

@@ -58,6 +58,7 @@ Other changes:
   a wrong viewport if none was initially set.
 - Backends: DirectX9: Using RGBA format when allowed by the driver to avoid CPU side
   conversion. (#6575) [@Demonese]
+- Internals: Fixed ImFileOpen not working before context is created. (#7314) [@PathogenDavid]
 
 
 -----------------------------------------------------------------------

imgui.cpp

@@ -2063,9 +2063,12 @@ ImFileHandle ImFileOpen(const char* filename, const char* mode)
     // Previously we used ImTextCountCharsFromUtf8/ImTextStrFromUtf8 here but we now need to support ImWchar16 and ImWchar32!
     const int filename_wsize = ::MultiByteToWideChar(CP_UTF8, 0, filename, -1, NULL, 0);
     const int mode_wsize = ::MultiByteToWideChar(CP_UTF8, 0, mode, -1, NULL, 0);
+    ImVector<char> local_temp;
+    // Fall back to own temp buffer if no context exists yet (EG: when creating shared font atlas before initializing.)
     ImGuiContext& g = *GImGui;
-    g.TempBuffer.reserve((filename_wsize + mode_wsize) * sizeof(wchar_t));
-    wchar_t* buf = (wchar_t*)(void*)g.TempBuffer.Data;
+    ImVector<char>& temp = &g != NULL ? g.TempBuffer : local_temp;
+    temp.reserve((filename_wsize + mode_wsize) * sizeof(wchar_t));
+    wchar_t* buf = (wchar_t*)(void*)temp.Data;
     ::MultiByteToWideChar(CP_UTF8, 0, filename, -1, (wchar_t*)&buf[0], filename_wsize);
     ::MultiByteToWideChar(CP_UTF8, 0, mode, -1, (wchar_t*)&buf[filename_wsize], mode_wsize);
     return ::_wfopen((const wchar_t*)&buf[0], (const wchar_t*)&buf[filename_wsize]);