block_canvas: Select the new block code node in add block code

This fixes an issue where the blocks picker incorrectly generates its
list of blocks before the block code node's block script is ready.

addons/block_code/ui/main_panel.gd

@@ -216,6 +216,7 @@ func _on_block_canvas_add_block_code():
 	undo_redo.create_action("Add block code for %s" % edited_node.name, UndoRedo.MERGE_DISABLE, edited_node)
 
 	undo_redo.add_do_method(edited_node, "add_child", block_code, true)
+	undo_redo.add_do_method(self, "_select_node", block_code)
 	undo_redo.add_do_property(block_code, "owner", scene_root)
 	undo_redo.add_do_property(_context, "block_code_node", block_code)
 	undo_redo.add_do_reference(block_code)
@@ -225,6 +226,11 @@ func _on_block_canvas_add_block_code():
 	undo_redo.commit_action()
 
 
+func _select_node(node: Node):
+	EditorInterface.get_selection().clear()
+	EditorInterface.get_selection().add_node(node)
+
+
 func _on_block_canvas_open_scene():
 	var edited_node: Node = EditorInterface.get_inspector().get_edited_object() as Node