Can't copy errors from the error box

[KoBeWi]

Godot version

c6f6c05

System information

W10

Issue description

Example error:

func _ready():
	var sprite = Sprite2D.new()
	sprite.material = CanvasItemMaterial.new()

Steps to reproduce

1. Make script error
2. Click this button at the bottom:
   
3. Try to copy contents

Minimal reproduction project

No response

[rsubtil]

Regression from #42595

The code inside RichTextLabel::_get_line_text is always returning an empty string despite the selection existing.

cc @bruvzg