Spawning enemies and trigger event

I have a scenario where I have a button that spawns in 6 demons that I've given each the TID "6" at at a mapspot that I have placed in my map. like this..
 

script  6 (void) 
{
	Floor_LowerToLowest(4,16);
	Light_ChangeToValue(6,148);
	delay(40);
	for(int demon = 0; demon <6;demon++)
	{
		delay(40);
		Thing_SpawnFacing(5,T_DEMON,FALSE, 6);
	}
}

first it lowers the floor that the button is on then sets the brightness of a sector, and then spawns the demons.  my trouble is that I want to make it so that if you kill all these demons a door opens up. However, because the demons don't exist anywhere in the map before they spawn the script below won't trigger.  I think that's what the problem is but I do not know for sure.

script 4 OPEN
{
	while (ThingCount(T_NONE,6)<0)
	{
		delay(30);
		Floor_LowerToNearest(5,16);
	}
}

If anyone has any idea how to fix this, let me know.

Try this (note the changes in both scripts, including the change in the type of script 4):

script  6 (void) 
{
	Floor_LowerToLowest(4,16);
	Light_ChangeToValue(6,148);
	delay(40);
	for(int demon = 0; demon <6;demon++)
	{
		delay(40);
		Thing_SpawnFacing(5,T_DEMON,FALSE, 6);
	}
	SetThingSpecial(6, ACS_ExecuteAlways, 4);
}
script 4 (void)
{
	if (ThingCount(T_NONE,6)==0)
	{
		delay(30);
		Floor_LowerToNearest(5,16);
	}
}

EDIT: I've just realized this code is not foolproof: If you kill the first 5 demons before the 6th one spawns, and the 6th one won't spawn due to something obstructing the landing spot, the Floor_LowerToNearest won't activate.

1 hour ago, scifista42 said:

Try this (note the changes in both scripts, including the change in the type of script 4):

script  6 (void) 
{
	Floor_LowerToLowest(4,16);
	Light_ChangeToValue(6,148);
	delay(40);
	for(int demon = 0; demon <6;demon++)
	{
		delay(40);
		Thing_SpawnFacing(5,T_DEMON,FALSE, 6);
	}
	SetThingSpecial(6, ACS_ExecuteAlways, 4);
}
script 4 (void)
{
	if (ThingCount(T_NONE,6)==0)
	{
		delay(30);
		Floor_LowerToNearest(5,16);
	}
}

EDIT: I've just realized this code is not foolproof: If you kill the first 5 demons before the 6th one spawns, and the 6th one won't spawn due to something obstructing the landing spot, the Floor_LowerToNearest won't activate. 

There is a problem I've been having with the demons spawning in, even without the kill condition.  I think it has something to do with the fact that they are all coming out of the same spot, and with a random rotation angle, which means from time to time one of the demons wont' see you and will block the next one from coming in.

You can actually check if a spot is free or big enough to spawn a monster by doing something along the lines of:

script 777 (int x, int y, int z)
{
	int tempTID = UniqueTID();
	int canSpawn = Spawn("Cacodemon", x, y, z, tempTID);
    
    if (canSpawn) // Checks the location given by x, y, & z
    {
    	Thing_Remove(tempTID);
        
        /*Put stuff here that you want to happen if the monster can spawn in the map*/
    }
    
    else
    {
    	/*If he can't? Do whatever you want here*/
    }
}

Adapt this into your already existing code however you may need.