Fix crashes with Qt 5.12

Commit 1ccd6815 moved the creation of the mNetworkAccessManager down
without a mentioned reason, but that apparently causes crashes with Qt
5.12 at least.

Moving it back to the top seems to fix that.

BUG: 371511
FIXED-IN: 5.11.2
Differential Revision: https://phabricator.kde.org/D21236