When Minyaa Time is installed, it apply a default value for Worklog Type fot all existing Wotklog.
If Worklog are created without using Minyaa Time feature :

• by disabling temporary Minyaa Time
• or by using other Worklog Creation procedures (like other plugins "Timesheet Reports and Gadgets", "Tempo", ...)
  ... then no Worklog Type are specified for these new entries.
  It causes a NPE when trying to edit them.