Accidentally pasting a codebox into a codebox causes problems

[Qehremanov]

Version, Operating system
CherryTree version: 1.2.0
OS: Ubuntu 20.04.1
Desktop: Gnome

Describe the bug
Multiple issues occur when I accidentally copy and paste a CodeBox into a Codebox.
For example, a codebox that I pasted into the codebox appears on the first line of the node. When I try to save the program, it crashes.

To Reproduce
Steps to reproduce the behavior:

1. Create a codebox. (Shift + Alt + D)
2. Write something in it.
3. Copy the codebox. (Ctrl + C)
4. Create a new codebox. (Shift + Alt + D)
5. Paste the codebox into the newly created codebox. (Ctrl + V)
6. You will see the old codebox in the first line of the node.
7. Delete the newly created codebox.
8. Save.
9. The program will crash.

Screenshots

[giuspen]

Thanks for reporting, this has now been fixed.