I've seen two main problems. The first is in line 19 of RichTextLabel.gd.
There you are trying to use
show() function from RichTextLabel node. However (besides you forgot the () in show), as the script is attached directly to RichTextLabel, you shouldn't try to get this:
but call directly to show:
or if you want to use full path:
The error isn't to hard to interpret once you start to know the engine a little. It says
Invalid get index 'show' (on base: 'null instance') because the sentence
get_tree().get_root().get_node("Polygon2D/RichTextLabel").show returns null as you are trying to get RichTextLabel from that full path, and you are missing Node2D. So the error says that the null returned has not an index called show.
Similar error is the one you have in the timer timeout's function. There you use:
but you are already inside RichTextLabel as the script is attached to it. So instead you should directly use:
The other things i see are that the text won't show anyways, because it's Polygon2D is not visible, and also you set to 0 visible characters in ready. You need to change that when you want the text to appear, but i assume you already know that.
Tell me if its helps! it works on my pc