crash correction

heli_control.lua

@@ -37,6 +37,9 @@ function helicopter.check_node_below(obj)
 end
 
 function helicopter.heli_control(self, dtime, touching_ground, liquid_below, vel_before)
+    if self.driver_name == nil then
+        return
+    end
 	local driver = minetest.get_player_by_name(self.driver_name)
 	if not driver then
 		-- there is no driver (eg. because driver left)

init.lua

@@ -435,8 +435,10 @@ minetest.register_entity("helicopter:heli", {
 			-- driver clicked the object => driver gets off the vehicle
 			self.driver_name = nil
 			-- sound and animation
-			minetest.sound_stop(self.sound_handle)
-			self.sound_handle = nil
+            if self.sound_handle then
+			    minetest.sound_stop(self.sound_handle)
+			    self.sound_handle = nil
+            end
 			self.object:set_animation_frame_speed(0)
 			-- detach the player
 			clicker:set_detach()