Hi Pert,
I installed the current hourly build from https://www.arduino.cc/en/Main/Software.
I ran that version and then invoked the boards manger in the Arduino IDE Tools->Board->Board Manager.
The boards manager update process ran OK and did not report any java errors or (red) error message.
I then proceeded to re-run IDE version 1.8.6 and and then invoked the boards manger in the Arduino IDE Tools->Board->Board Manager.
The boards manager update process ran OK and did not report any java errors or (red) error message.
Peter