How to Fix Problems with injecting DLLs
To avoid constant questions in these mod threads, I've compiled a list of fixes for problems with injecting dlls (including one I just found out).
Multiple DLLs
It just doesn't work, don't bother asking. Inject single DLLs.
Start off by using Extreme Injector v3.6.1 (Link)
I recommend using extreme injector as it is very efficient. And open the process "GTA5.exe" by clicking Select. (or just type in GTA5.exe and it will auto-detect)
Make sure you have these settings saved:
Inject Times
It is recommended you inject the DLLs in the first main menu, story mode, or the terms of agreement black screen (only works sometimes).
Just refrain from injecting as you start GTA V (no auto injecting!) and Online.
Install the Visual C++ Packages
Without the C++ packages, the mods will most likely not work. Install as many of them (latest mostly), and the 2015 version especially (both x86 amd x64, including everything else also being x86 and x64). It is also better you have .NET Framework 4.6. [I might upload a compiled .rar of all of these, for now, google.]
Keep the DLLs and injectors out of the game's folder
This may come very obvious. Just don't put any kind of mod files right into the GTA folder, atleast one folder before it is fine.
Have a clean GTA5.exe
Errors like "returned NULL" occur because you do not have a clean version of the GTA executable (for some reason).
You most likely have edited the .exe to remove the intro videos/splash screens, and this affects injecting DLLs.
Revert back by Ctrl+F "rockstar_log" or anything similar to what you changed it to and edit it back to "rockstar_logos". (open GTA5.exe with notepad, preferably Notepad++ masterace).
MSVCPR DLL or w/e
There was a tiny issue with the injectors saying a particular (msvcpr[versionname]) is missing. Iirc, extreme injector notifies you of this and allows you to download it directly by the push of a button (yay extreme injector!). If you didn't get a prompt like that, google the dll name, ex:- msvcpr110.dll, and download the dll (usually the first link, there's a famous site for dlls), and put it in C:\Windows\System32.
And that's it! If you have any other problems or fixes, please share. Hope this helped!