add child node assertions to units

scripts/units/controlled_unit.gd

@@ -9,6 +9,9 @@ var ground_plane: Plane = Plane(Vector3.UP, 0)
 
 
 func _ready() -> void:
+	assert(camera != null, "camera missing!")
+	assert(selection_sprite != null, "selection_sprite missing!")
+
 	set_selected(false)
 	super._ready()
 

scripts/units/unit.gd

@@ -16,6 +16,11 @@ var is_on_screen: bool = false
 
 
 func _ready() -> void:
+	assert(hover_sprite != null, "hover_sprite missing!")
+	assert(nav_agent != null, "nav_agent missing!")
+	assert(animation_player != null, "animation_player missing!")
+	assert(visibility_notifier != null, "visibility_notifier missing!")
+
 	set_hovered(false)
 	nav_agent.max_speed = MOVE_SPEED
 	nav_agent.velocity_computed.connect(_on_nav_agent_velocity_computed)