JabRef 5.0-beta.419--2020-02-12--3859620
Linux 5.4.19-050419-generic amd64
Java 13.0.2
Steps to reproduce the behavior:
- Click on the symbol to open a PDF linked to a bib entry (or use F4 or use the "Open menu" command from the context menu).
- The PDF viewer (in my case MasterPDFEditor is associated with application/pdf) opens.
- JabRef can only be used after the PDF viewer is closed again (in the screenshot JabRef is not properly rendered because the whole application is blocked).

I first thought this only happens in my i3/Gnome Flashback environment. But switching to plain Ubuntu 19.10 Gnome desktop, exactly the same behaviour can be observed.
This phenomenon didn't appear for me (with highly similar settings) in JabRef 4.x.
I am seeing this exception on the console after closing JabRef, though this might not result from having called an external program (I suppose):
Log File
Exception in Application stop method
ERROR FallbackExceptionHandler Uncaught exception occurred in Thread[main,5,main]
java.lang.RuntimeException: Exception in Application stop method
at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source)
at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)
Caused by: java.lang.IllegalAccessError: class kong.unirest.JsonObjectMapper (in module org.jabref.merged.module) cannot access class com.google.gson.Gson (in module com.google.gson) because module org.jabref.merged.module does not read module com.google.gson
at org.jabref.merged.module/kong.unirest.JsonObjectMapper.(Unknown Source)
at org.jabref.merged.module/kong.unirest.Config.(Unknown Source)
at org.jabref.merged.module/kong.unirest.Unirest.(Unknown Source)
at org.jabref/org.jabref.Globals.stopBackgroundTasks(Unknown Source)
at org.jabref/org.jabref.JabRefMain.stop(Unknown Source)
at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$10(Unknown Source)
at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(Unknown Source)
at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$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$11(Unknown Source)
... 1 more
JabRef 5.0-beta.419--2020-02-12--3859620
Linux 5.4.19-050419-generic amd64
Java 13.0.2
Steps to reproduce the behavior:
I first thought this only happens in my i3/Gnome Flashback environment. But switching to plain Ubuntu 19.10 Gnome desktop, exactly the same behaviour can be observed.
This phenomenon didn't appear for me (with highly similar settings) in JabRef 4.x.
I am seeing this exception on the console after closing JabRef, though this might not result from having called an external program (I suppose):
Log File
Exception in Application stop method ERROR FallbackExceptionHandler Uncaught exception occurred in Thread[main,5,main] java.lang.RuntimeException: Exception in Application stop method at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.launchApplication1(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.lambda$launchApplication$2(Unknown Source) at java.base/java.lang.Thread.run(Unknown Source) Caused by: java.lang.IllegalAccessError: class kong.unirest.JsonObjectMapper (in module org.jabref.merged.module) cannot access class com.google.gson.Gson (in module com.google.gson) because module org.jabref.merged.module does not read module com.google.gson at org.jabref.merged.module/kong.unirest.JsonObjectMapper.(Unknown Source) at org.jabref.merged.module/kong.unirest.Config.(Unknown Source) at org.jabref.merged.module/kong.unirest.Unirest.(Unknown Source) at org.jabref/org.jabref.Globals.stopBackgroundTasks(Unknown Source) at org.jabref/org.jabref.JabRefMain.stop(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.LauncherImpl.lambda$launchApplication1$10(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$runAndWait$12(Unknown Source) at org.jabref.merged.module/com.sun.javafx.application.PlatformImpl.lambda$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) ... 1 more