I'm working on a somewhat complex editor plug-in and began encountering errors in the console window such as the following
_ERROR: Parse Error: Busy. [Resource file res://addons/path_to_my_plugin/my_plugin_main_screen.tscn:197]
at: parse_node_tag (scene/resources/resource_format_text.cpp:289)
ERROR: Failed loading resource: res://addons/path_to_my_plugin/my_plugin_main_screen.tscn:197. Make sure resources have been imported by opening the project in the editor at least once.
at: (core/io/resource_loader.cpp:283)
I eventually realized the errors occur only when loading the project with the editor plugin disabled. No errors occur when loading the project with the plugin enabled. The problem is caused by the plugin's @tool scripts being instantiated by the editor during parsing without the context established by the plug-in script.
Not sure if this is a bug or a fact of life when building editor plugins.