I tested it for two versions:

JabRef 5.0--2020-03-06--2e6f433
Linux 5.6.0-1-amd64 amd64
Java 13.0.2. -->
and
JabRef 5.1--2020-05-29--ed275b6
Linux 5.6.0-1-amd64 amd64
Java 14.0.1

• Mandatory: I have tested the latest development version from http://builds.jabref.org/master/ and the problem persists

Steps to reproduce the behavior:
when I turn on linux, the Jabref work perfectly but when I close it and I tried to rerun it the follow error appear:

java.lang.NullPointerException: You need to start the file monitor before watching files at java.base/java.util.Objects.requireNonNull(Unknown Source) at org.jabref/org.jabref.gui.util.DefaultFileUpdateMonitor.addListenerForFile(Unknown Source) at org.jabref/org.jabref.gui.collab.DatabaseChangeMonitor.lambda$new$0(Unknown Source) at java.base/java.util.Optional.ifPresent(Unknown Source) at org.jabref/org.jabref.gui.collab.DatabaseChangeMonitor.<init>(Unknown Source) at org.jabref/org.jabref.gui.BasePanel.resetChangeMonitorAndChangePane(Unknown Source) at org.jabref/org.jabref.gui.BasePanel.setupMainPanel(Unknown Source) at org.jabref/org.jabref.gui.BasePanel.<init>(Unknown Source) at org.jabref/org.jabref.gui.importer.actions.OpenDatabaseAction.addNewDatabase(Unknown Source) at org.jabref/org.jabref.gui.importer.actions.OpenDatabaseAction.lambda$openTheFile$1(Unknown Source) at org.jabref/org.jabref.gui.util.DefaultTaskExecutor.lambda$getJavaFXTask$3(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.CompositeEventHandler.dispatchBubblingEvent(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.EventHandlerManager.dispatchBubblingEvent(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.BasicEventDispatcher.dispatchEvent(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.EventDispatchChainImpl.dispatchEvent(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.EventUtil.fireEventImpl(Unknown Source) at org.jabref.merged.module/com.sun.javafx.event.EventUtil.fireEvent(Unknown Source) at org.jabref.merged.module/javafx.event.Event.fireEvent(Unknown Source) at org.jabref.merged.module/javafx.concurrent.EventHelper.fireEvent(Unknown Source) at org.jabref.merged.module/javafx.concurrent.Task.fireEvent(Unknown Source) at org.jabref.merged.module/javafx.concurrent.Task.setState(Unknown Source) at org.jabref.merged.module/javafx.concurrent.Task$TaskCallable.lambda$call$1(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(Unknown Source) at java.base/java.security.AccessController.doPrivileged(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(Unknown Source) at org.jabref.merged.module/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(Unknown Source) at org.jabref.merged.module/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method) at org.jabref.merged.module/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(Unknown Source) at java.base/java.lang.Thread.run(Unknown Source)

Paste an excerpt of your log file here