FLUID-5214: removed default sourceApplier

Replaced the default sourceApplier: null with a comment about how to supply one. This default had lead to the possibility of the sourceApplier being overridden by the default null value depending on grade name order.
Merge branch 'FLUID-5214'

* FLUID-5214:
  FLUID-5214: removed default sourceApplier
Merge branch 'master' into FLUID-5213