SUMMARY
The crash is reproductible through 19.12-3, 20.04, 20.07.70 releases  (and many previous releases)

STEPS TO REPRODUCE
1. Open an existing project
2. Move to the timeline
3. Try to alter/modify a transform effect (or a couple of parameters)

OBSERVED RESULT
random kdenlive crash , prompting following outputs :

qml: +++++++++++++++++++ DRAG CLICKED +++++++++++++
-----
// SHOW KEYFRAMES:  true
MUTEX LOCK!!!!!!!!!!!! setmodel
MUTEX UNLOCK!!!!!!!!!!!! setmodel
MUTEX LOCK!!!!!!!!!!!! loadEffects COUNT:  1
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
// FOUND ADDED PARAM
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
===================================================== Requested rowCount QModelIndex(-1,-1,0x0,QObject(0x0)) 5
// // // RESULTING REQUIRED SCENE:  2
MUTEX UNLOCK!!!!!!!!!!!! loadEffects
-----
// SHOW KEYFRAMES:  true
update lambda 0 QVariant(QString, "-382 -215 19 216 1") true
 = = SET EFFECT PARAM:  "rect"  =  "0=-382 -215 19 216 1;60=480 27 960 540 1"
ADD keyframe 78 QVariant(double, 0) true
add lambda 78 QVariant(double, 0) true
 = = SET EFFECT PARAM:  "rotation"  =  "0=0;60=0;78=0"
ADD keyframe 78 QVariant(QString, "480 27 960 540 1") true
add lambda 78 QVariant(QString, "480 27 960 540 1") true
 = = SET EFFECT PARAM:  "rect"  =  "0=-382 -215 19 216 1;60=480 27 960 540 1;78=480 27 960 540 1"
QPainter::begin: Paint device returned engine == 0, type: 3
QPainter::setCompositionMode: Painter not active
QPainter::setRenderHint: Painter must be active to set rendering hints
QPainter::setWorldTransform: Painter not active
QPainter::setOpacity: Painter not active
QPainter::end: Painter not active, aborted
ASSERT: "mImg == qImg->constBits()" in file common.cpp, line 63
/tmp/.mount_kdenliAepfam/AppRun : ligne 26 : 10115 Abandon                 (core dumped) kdenlive --config kdenlive-appimagerc $@


EXPECTED RESULT
No crash, just modifying the parameter (or prompting an alert for an invalid parameter)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Ubuntu 19.10
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION
AppImage releases