Fix updating about data with custom functions

Since we are in the constructor of the Aboutdata object
calling KABoutData::applicationData results in an empty
object and setting it will later be overwritten.

Fixes commit: 09564b2613ac80834156c879bd15d2498a824b6d

(cherry picked from commit 00986a73415fba67e060c031704188f793e8b633)

• GnuPG-Bug-Id: T6797