Checked whether multiple enabled navpolys upon adding and deleting

Impossible to check upon navpoly enabling without creating a new script/custom node.

addons/escoria-core/game/core-scripts/esc_terrain.gd

@@ -64,8 +64,36 @@ onready var logger = Logger.ESCLoggerFile.new()
 # Set a reference to the active navigation polygon, register to Escoria
 # and update the texture
 func _ready():
+	connect("child_entered_tree", self, "_check_multiple_enabled_navpolys")
+	connect("child_exiting_tree", self, "_check_multiple_enabled_navpolys", [true])
+	
+	_check_multiple_enabled_navpolys()
+	if !Engine.is_editor_hint():
+		escoria.room_terrain = self
+	_update_texture()
+
+
+# Checks whether multiple navigation polygons are enabled.
+# Shows a warning in the terminal if this happens.
+# TODO: change this "simple" console log for an editor warning
+# by overriding Node._get_configuration_warning() after we get rid of 
+# deprecated Navigation2D. 
+#
+# #### Parameters
+#
+# - node: if this method is triggered by child_entered_tree or 
+# child_exited_tree signals, parameter is the added node.
+func _check_multiple_enabled_navpolys(node: Node = null, is_exiting: bool = false) -> void:
 	var navigation_enabled_found = false
+	if node != null \
+			and not is_exiting\
+			and node is NavigationPolygonInstance \
+			and node.enabled:
+		 navigation_enabled_found = true
+	
 	for n in get_children():
+		if is_exiting and n == node:
+			continue
 		if n is NavigationPolygonInstance and n.enabled:
 			if navigation_enabled_found:
 				if Engine.is_editor_hint():
@@ -80,15 +108,11 @@ func _ready():
 						"Multiple NavigationPolygonInstances enabled " + \
 						"at the same time."
 					)
+				return
 			else:
 				navigation_enabled_found = true
 				current_active_navigation_instance = n
-
+	
-
-	if !Engine.is_editor_hint():
-		escoria.room_terrain = self
-	_update_texture()
-
 	
 # Return the Color of the lightmap pixel for the specified position
 #