Color blocks produce errors when deleted

[stephanemagnenat]

The following errors are produced when a top-color palette block is deleted:

qrc:/thymio-vpl2/blocks/PaletteTopColorActionBlock.qml:52: TypeError: Cannot read property of null
qrc:/thymio-vpl2/blocks/PaletteTopColorActionBlock.qml:53: TypeError: Cannot read property of null
file:///home/steph/test/Qt/5.9.4/gcc_64/qml/QtGraphicalEffects/RadialGradient.qml:373:4: QML ShaderEffectSource: Cannot anchor to an item that isn't a parent or sibling.
file:///home/steph/test/Qt/5.9.4/gcc_64/qml/QtGraphicalEffects/RadialGradient.qml:383:5: QML ShaderEffect: Cannot anchor to an item that isn't a parent or sibling.
file:///home/steph/test/Qt/5.9.4/gcc_64/qml/QtGraphicalEffects/RadialGradient.qml:373:4: QML ShaderEffectSource: Cannot anchor to an item that isn't a parent or sibling.
file:///home/steph/test/Qt/5.9.4/gcc_64/qml/QtGraphicalEffects/RadialGradient.qml:383:5: QML ShaderEffect: Cannot anchor to an item that isn't a parent or sibling.

Similar errors arise with all color-related blocks.