Can you reproduce this in Troubleshooting Mode (Help → "Restart in Troubleshooting Mode…”), which temporarily disables all plugins?

Can you provide a Debug ID for it happening, but this time in Troubleshooting Mode to make sure all plugins are disabled?

All your plugins are still active. Please provide a Debug ID with all third-party plugins disabled to ensure the issue isn’t caused by a plugin.

If a plugin is responsible, you’ll need to report the issue to its developers.

This time I manually disabled all plugins, and the problem still occurred. Debug ID: 523210009.

@martynas_b: Steps to reproduce:
1) Open a PDF file in the Zotero PDF Reader.
2) Close the left pane (listing the annotations)
3) Make an highlight in a PDF file
4) Click on the highlight, so that the comment popup comes out
5) Right-click on the highlight -> Convert to Underline
6) Click on the highlight, so that the comment popup comes out
7) Right-click on the highlight -> Convert to Highlight
8) Eventually repeat steps 4-7, or make them fast enough.
-> The toolbar has disappeared.

[JavaScript Error: "annotation is undefined" {file: "resource://zotero/reader/reader.js" line: 25346}]
[JavaScript Error: "toolbarRef.current is null" {file: "resource://zotero/reader/reader.js" line: 22760}]

Debug ID D431212817
Zotero 7.1-beta.14+2e9b9d407 (64-bit)
Windows 10

Thanks for your efforts!

The problem has been solved for me! Thank you for your quick response and efficient handling.