Currently, adding new objects/components to the FixedSlotSequence PST is
not being checked, meaning that it is possible to add NULL objects here
without any warning. This causes NULL-pointer errors when non-existent
components are added, which can be hard to debug.
To solve this, add a check for the object existence before adding it to
PST and emit an error message.
Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
