Error: Cannot read property 'message' of null

Hello,
everytime I click on “Show received messages” button on the System/Inputs page I get this
error: Cannot read property ‘message’ of null

Graylog version 3.0.2
Elasticsearch 6.6.0

Any idea whats wrong?

Update: It happens also when I click on a Stream. All queries seem to fail now.

check you logfiles of Graylog and Elasticsearch.