Some QML properties access exposed Python objects that may or may not
be null upon their access. When these objects are accessed while null,
QML issues "TypeError" warnings. These warnings have no functional
impact as QML correctly handles trying to access null objects, but can
spam the logs.
This commit aims at fixing all these warnings by checking that the
Python objects are not null before being accessed.
* display and allow to upgrade incompatible nodes after project loading
* add a ToolButton in main toolbar to open CompatibilityManager in case of compatibility issues
