Tool Activation prevention

[penpendrum]

I have a plugin that i was trying to make and I do not want it to start when I have not selected a group. I thought this solution would work but it does not.
below is some part of my code . any tip would be greatly appreciated.

class Testing

 
  def initialize
    @ip = nil
    @ip1 = nil
    @ip2 = nil
    @xdown = 0
    @ydown = 0
    @state = 0
  end
  
  def activate
     # I was expecting it not to activate when nothing is selected at first
     sel = Sketchup.active_model.selection
     break if sel.empty?
     
     # some codes
  end

  def onMouseMove(flags, x, y, view)
    # some codes
  end

end