When I start kde setting then go to Application style and then clicking on Window decorations the setting window crashes with the following errors, this error is always reproducible on my machine.
I use :
Operating System: Kubuntu 19.10
KDE Plasma Version: 5.17.4
KDE Frameworks Version: 5.64.0
Qt Version: 5.12.4
Kernel Version: 5.3.0-25-generic
OS Type: 64-bit
Processors: 2 × Pentium® Dual-Core CPU T4400 @ 2.20GHz
Memory: 5.6 GiB of RAM

The error is :
QQmlEngine::setContextForObject(): Object already has a QQmlContext
kf5.kactivity.stat: [Error at ResultSetPrivate::initQuery]:  QSqlError("1", "Unable to execute statement", "no such column: rl.initiatingAgent")
kf5.kactivity.stat: [Error at ResultSetPrivate::initQuery]:  QSqlError("1", "Unable to execute statement", "no such column: rl.initiatingAgent")
org.kde.kcoreaddons: Error loading plugin "kcm_desktoptheme" "The shared library was not found." 
Plugin search paths are ("/usr/lib/x86_64-linux-gnu/qt5/plugins", "/usr/bin") 
The environment variable QT_PLUGIN_PATH might be not correctly set
QQmlEngine::setContextForObject(): Object already has a QQmlContext
qt.svg: <input>:663: Could not resolve property: #radialGradient3118
qt.svg: <input>:663: Could not resolve property: #radialGradient3112
qt.svg: <input>:663: Could not resolve property: #radialGradient3327
qt.svg: <input>:663: Could not resolve property: #radialGradient3085
qt.svg: <input>:663: Could not resolve property: #radialGradient3101
qt.svg: <input>:663: Could not resolve property: #radialGradient3139
qt.svg: <input>:663: Could not resolve property: #radialGradient3133
qt.svg: <input>:663: Could not resolve property: #radialGradient3499
org.kde.kcoreaddons: Error loading plugin "kcm_lookandfeel" "The shared library was not found." 
Plugin search paths are ("/usr/lib/x86_64-linux-gnu/qt5/plugins", "/usr/bin") 
The environment variable QT_PLUGIN_PATH might be not correctly set
QQmlEngine::setContextForObject(): Object already has a QQmlContext
org.kde.kcoreaddons: Error loading plugin "kcm_desktoptheme" "The shared library was not found." 
Plugin search paths are ("/usr/lib/x86_64-linux-gnu/qt5/plugins", "/usr/bin") 
The environment variable QT_PLUGIN_PATH might be not correctly set
QQmlEngine::setContextForObject(): Object already has a QQmlContext
qt.svg: <input>:663: Could not resolve property: #radialGradient3118
qt.svg: <input>:663: Could not resolve property: #radialGradient3112
qt.svg: <input>:663: Could not resolve property: #radialGradient3041
qt.svg: <input>:663: Could not resolve property: #radialGradient3079
qt.svg: <input>:663: Could not resolve property: #radialGradient3073
qt.svg: <input>:663: Could not resolve property: #radialGradient3499
qt.svg: <input>:663: Could not resolve property: #radialGradient3505
qt.svg: <input>:663: Could not resolve property: #radialGradient3118
qt.svg: <input>:663: Could not resolve property: #radialGradient3112
qt.svg: <input>:663: Could not resolve property: #radialGradient3118
org.kde.kcoreaddons: Error loading plugin "kcm_kwindecoration" "The shared library was not found." 
Plugin search paths are ("/usr/lib/x86_64-linux-gnu/qt5/plugins", "/usr/bin") 
The environment variable QT_PLUGIN_PATH might be not correctly set
QQmlEngine::setContextForObject(): Object already has a QQmlContext
Module 'org.kde.kwin.decoration' does not contain a module identifier directive - it cannot be protected from external registrations.
KCrash: crashing... crashRecursionCounter = 2
KCrash: Application Name = systemsettings5 path = /usr/bin pid = 14743
KCrash: Arguments: /usr/bin/systemsettings5 
KCrash: Attempting to start /usr/lib/x86_64-linux-gnu/libexec/drkonqi from kdeinit
sock_file=/run/user/1000/kdeinit5__0
QSocketNotifier: Invalid socket 8 and type 'Read', disabling...

[1]+  Stopped                 systemsettings5