Solved: I just installed the oracle jre version.

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Running with the following parameters - headless: false, libraryport: -1, adminport: -1, remoteadmin: false, host: null, debug: false, execdir: /home/casimoes/Downloads

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - #############################

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - # Starting Ubooquity #

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - #############################

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Java version: 1.8.0_191

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Java vendor: Oracle Corporation

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - java vm name: OpenJDK 64-Bit Server VM

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - java runtime name: OpenJDK Runtime Environment

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Max heap size available: 1748 MB

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - OS name: Linux

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - OS version: 4.18.0-18-generic

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - OS architecture: amd64

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Loading local version info

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Ubooquity 2.1.2 built on 2018-10-14 at 12:58

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Creating application folders

20190511 09:47:59 [main] INFO com.ubooquity.Ubooquity - Loading preferences

20190511 09:48:02 [main] INFO org.eclipse.jetty.util.log - Logging initialized @7072ms to org.eclipse.jetty.util.log.Slf4jLog

20190511 09:48:02 [Thread-2] INFO com.ubooquity.b - Stopping internal server...

20190511 09:48:02 [Thread-2] INFO com.ubooquity.data.feeder.a - Aborting content scan...

20190511 09:48:02 [Thread-2] ERROR com.ubooquity.b - Error while stopping internal server

java.lang.NullPointerException: null

at com.ubooquity.d.e.a(SourceFile:99) ~[Ubooquity.jar:2.1.2]

at com.ubooquity.b.c(SourceFile:112) ~[Ubooquity.jar:2.1.2]

at com.ubooquity.Ubooquity.C(SourceFile:216) [Ubooquity.jar:2.1.2]

at java.lang.Thread.run(Thread.java:748) ~[na:1.8.0_191]

20190511 09:48:02 [Thread-2] INFO com.ubooquity.b - Internal server stopped

20190511 09:48:02 [Thread-2] INFO com.ubooquity.Ubooquity - Ubooquity has closed.

Can you help me? I'm a new linux user.