Fix inspector plugin leak

Handling the inspector plugin as a static variable of the BlockCode
class means it can never be removed from inspector since we never know
when all BlockCode instances have been destroyed. That means it had to
be leaked as shown in the verbose Godot output:

WARNING: ObjectDB instances leaked at exit (run with --verbose for details).
     at: cleanup (core/object/object.cpp:2209)
Leaked instance: EditorPlugin:1833129019089 - Node name:

Instead, add it once from the plugin entry point. That means it can also
be removed when the plugin exits rather than being leaked. This follows
the pattern documented in the Inspector Plugin tutorial[1].

1. https://docs.godotengine.org/en/stable/tutorials/plugins/editor/inspector_plugins.html.

Author: dbnicholson

addons/block_code/block_code_node/block_code.gd

@@ -4,7 +4,6 @@ class_name BlockCode
 extends Node
 
 @export var block_script: BlockScriptData = null
-static var plugin
 
 
 func _ready():
@@ -31,10 +30,6 @@ func _enter_tree():
 		new_bsd.generated_script = new_bsd.generated_script.replace("INHERIT_DEFAULT", new_bsd.script_inherits)
 		block_script = new_bsd
 
-	if plugin == null:
-		plugin = ClassDB.instantiate("EditorPlugin")
-		plugin.add_inspector_plugin(load("res://addons/block_code/inspector_plugin/block_script_inspector.gd").new())
-
 
 func _update_parent_script():
 	if Engine.is_editor_hint():

addons/block_code/block_code_plugin.gd

@@ -6,6 +6,9 @@ const MainPanel := preload("res://addons/block_code/ui/main_panel.tscn")
 static var main_panel: MainPanel
 static var block_code_button: Button
 
+const BlockInspectorPlugin := preload("res://addons/block_code/inspector_plugin/block_script_inspector.gd")
+var block_inspector_plugin: BlockInspectorPlugin
+
 var editor_inspector: EditorInspector
 var editor_selection: EditorSelection
 
@@ -52,6 +55,8 @@ func _enter_tree():
 	main_panel = MainPanel.instantiate()
 	main_panel.undo_redo = get_undo_redo()
 	block_code_button = add_control_to_bottom_panel(main_panel, _get_plugin_name())
+	block_inspector_plugin = BlockInspectorPlugin.new()
+	add_inspector_plugin(block_inspector_plugin)
 
 	# Remove unwanted class nodes from create node
 	old_feature_profile = EditorInterface.get_current_feature_profile()
@@ -71,6 +76,8 @@ func _enter_tree():
 
 
 func _exit_tree():
+	remove_inspector_plugin(block_inspector_plugin)
+
 	if block_code_button:
 		remove_control_from_bottom_panel(main_panel)
 		block_code_button = null