[vikarti]

I also can reproduce this. macOS Catalina, mac version of Calibre 4.9

calibre, version 4.9.0
ОШИБКА: Необработанное прерывание:: <b>TypeError</b>:unbound method toPlainText() must be called with PlainTextEdit instance as first argument (got QPlainTextEdit instance instead)

calibre 4.9 embedded-python: True is64bit: True
Darwin-19.2.0-x86_64-i386-64bit Darwin ('64bit', '')
('Darwin', '19.2.0', 'Darwin Kernel Version 19.2.0: Sat Nov 9 03:47:04 PST 2019; root:xnu-6153.61.1~20/RELEASE_X86_64')
Python 2.7.16
OSX: ('10.15.2', ('', '', ''), 'x86_64')
Interface language: ru
Successfully initialized third party plugins: Gather KFX-ZIP (from KFX Input) (1, 27, 0) && DeDRM (6, 6, 1) && Package KFX (from KFX Input) (1, 27, 0) && Manage Series (1, 2, 9) && Fantastic Fiction Adults (1, 1, 0) && Ex Libris (1, 0, 19) && MultiColumnSearch (1, 0, 79) && Modify ePub (1, 4, 0) && User Category (1, 5, 4) && BookFusion Plugin (0, 3, 2) && KFX metadata reader (from KFX Input) (1, 27, 0) && KFX Input (1, 27, 0) && X-Ray Creator (3, 2, 1) && Open With (1, 5, 10) && Count Pages (1, 9, 0) && EpubSplit (2, 8, 0) && Fantastic Fiction (1, 3, 0) && Webscription (1, 1, 0) && Reading List (1, 6, 7) && KindleUnpack - The Plugin (0, 82, 1) && Overdrive Link (2, 38, 0) && Find Duplicates (1, 6, 3) && Resize Cover (1, 0, 2) && Goodreads (1, 4, 0) && FanFicFare (3, 15, 0) && Save To Format (0, 0, 6) && Drop Search Results (1, 0, 10) && FictionDB (1, 2, 0) && Smashwords Metadata (1, 0, 0) && Quality Check (1, 9, 11) && Generate Cover (1, 5, 22) && Save Composite Custom Columns (1, 1, 0) && Read MP3 AudioBook metadata (1, 0, 79) && Wiki Reader (2, 1, 1) && Goodreads Sync (1, 13, 0) && EpubMerge (2, 8, 0) && Overdrive Link* (2, 38, 0)
Traceback (most recent call last):
File "site-packages/calibre/gui2/comments_editor.py", line 1142, in change_tab
TypeError: unbound method toPlainText() must be called with PlainTextEdit instance as first argument (got QPlainTextEdit instance instead)

Calibre bug report for this bug https://bugs.launchpad.net/calibre/+bug/1860666