How do I return a nil if a limb is already blown off?

This script is a landmine script. It requires my r15 ragdoll script. The landmine would launch players' bodies up in the air and blow off limbs by chance. The script would select any random children in the player's character ten times and checks if it's the right limb (e.g. RightUpperArm). The script works fine. However, if it tries to remove a limb's joint that's already been removed, it would error: RagdollRightUpperArm is not a valid member of Model I tried to fix my problem on line 24. But it doesn't work.

touch = true

function onTouch(PART)
	if touch then
		touch = false
		if touch == true then return end
		local humanoid = PART.Parent:FindFirstChild("Humanoid")
		if humanoid ~= nil then
			math.randomseed(tick())
			local sound = script.Parent:WaitForChild("Boom")
			sound:Play()
			humanoid.Health = 0
			PART.Velocity = Vector3.new(math.random(1,10), 500, math.random(1,10))
			PART.RotVelocity = Vector3.new(math.random(10,25), 0, math.random(10,25))
			local function chance(percentage)
    			return math.random() < percentage / 100
			end
			if chance(60) then
				for i = 1, 10 do
					local children = humanoid.Parent:GetChildren()
					wait(0.001)
					local removeJoint = (children[math.random(1,#children)])
					if removeJoint.Name == "RightUpperLeg" or removeJoint.Name == "RagdollRightUpperLeg" then
						if humanoid.Parent.RagdollRightUpperLeg == nil then
							print("Right leg already blown off!")
						end
						humanoid.Parent.RagdollRightUpperLeg:Destroy()
						print("Right leg blown off!")
					end
					if removeJoint.Name == "RightUpperArm" or removeJoint.Name == "RagdollRightUpperArm" then
						if humanoid.Parent.RagdollConstraintRightUpperArm == nil then
							print("Right arm already blown off!")
						end
						humanoid.Parent.RagdollRightUpperArm:Destroy()
						print("Right arm blown off!")
					end
					if removeJoint.Name == "LeftUpperLeg" or removeJoint.Name == "RagdollLeftUpperLeg" then
						if humanoid.Parent.RagdollLeftUpperLeg == nil then
							print("Left leg already blown off!")
						end
						humanoid.Parent.RagdollLeftUpperLeg:Destroy()
						print("Left leg blown off!")
					end
					if removeJoint.Name == "LeftUpperArm" or removeJoint.Name == "RagdollLeftUpperArm" then
						if humanoid.Parent.RagdollLeftUpperArm == nil then
							print("Left arm already blown off!")
						end
						humanoid.Parent.RagdollLeftUpperArm:Destroy()
						print("Left arm blown off!")
					end
				end
			else
  				print("No limbs blown off.")
			end
		end
		wait(1)
		touch = true
	end
end

script.Parent.Touched:connect(onTouch)