I believe you are using "Stop The Ped", correct?
If you check the .ini there is a system called "Realistic Weapon System". You can disable this and the problem disappears, but before you do so, read below.
The point of the realistic weapon system is to add realism, so if you are looking for more realism, leave the system enabled.
Reason: Even though you can not spawn in weapons, you can grab either a rifle or a shotgun from inside the vehicle by pressing G and press "grab rifle" or "grab shotgun". You can also edit which rifle and shotgun you get in the .ini.
You can also grab it from outside the vehicle by pressing G near the trunk, you get granted the same options here, but here you can also grab a fire extinguishers in case you are a first responder to a fire.
Pressing G on the trunk is also how you store evidence, i.e weapons dropped by the criminals.
I suggest everyone to read the .ini files that come along with mods to get a clear view of how the systems work, and also your options in-game, keybindings and such.