Entity Discovery, LUA error after a few tries.

[vortexnl]

My goal was to make an expression that if you spawn an explosive barrel, it flies towards the owner, and probaly kills him.
This is my script:

@name Explosive Barrel Killer
@inputs Button
@outputs 
@persist Barrel:entity Owner:entity Pos:vector Vector:vector 
runOnTick(1)
if(Button){
    findByModel("models/props_c17/oildrum001_explosive.mdl")
}
Barrel=findClosest(entity():pos())
Owner=Barrel:owner()
Pos=Owner:pos()
Vector=Pos-Barrel:massCenter()
Barrel:applyForce(Vector*100)

After 2 barrels I get this error:

Expression 2: entities\gmod_wire_expression2\core\find.lua:672: Tried to use a NULL entity!

Is my expression wrong? Please correct me! :O

EDIT: I kinda fixed it, I got rid of the runOnTick(1) and changed it with interval(100).
Is there an other way to fix this? Maybe by letting it search based on a timer?