NoClassDefFoundError when running a Scala/Swing program with sbt - scala

Question

I am trying to compile an old school project from around March, 2015. I haven’t touched it since then. The project is written in Scala with the Swing library, and the build tool is sbt.
Apparently my build.sbt file needs updating. After digging the Internet, I found that this configuration should work.
lazy val root = (project in file (".")).
settings (
name := "MyProgram",
/* FORMERLY: */
//libraryDependencies += "org.scala-lang" % "scala-swing" % "2.10+",
//ivyScala := ivyScala.value map { _.copy(overrideScalaVersion = true) }
/* NOW: */
scalaVersion := "2.13.0",
libraryDependencies += "org.scala-lang.modules" %% "scala-swing" % "2.1.1"
)
The trick would be to use compatible versions of Scala and Swing. I tried the following pairs, which I expected to work together (either because I saw them on the webpage of package Swing, or because it worked for other people on SO):
(Scala 2.13.0, Swing 2.1.1)
(Scala 2.12.6, Swing 2.1.1)
(Scala 2.12.1, Swing 2.0.0-M)
(Scala 2.11.1, Swing 1.0.2)
I also tried switching from Java 12 (jdk-openjdk 12.0.2.u10-1) to Java 8 (jdk8-openjdk 8.u222-2).
However, all combinations give the same error. Compiling (sbt compile) works fine, but running the application (sbt run) results in a bunch of NoClassDefFoundError / ClassNotFoundException errors:
$ sbt run
[info] Loading project definition from /home/me/MyProject/project
[info] Loading settings for project root from build.sbt ...
[info] Set current project to MyProject (in build file:/home/me/MyProject/)
[info] Compiling 22 Scala sources to /home/me/MyProject/target/scala-2.12/classes ...
[warn] there were four deprecation warnings (since 2.12.0); re-run with -deprecation for details
[warn] one warning found
[info] running Main.Main
[error] java.lang.NoClassDefFoundError: scala/swing/event/WindowOpened
[error] at scala.swing.Window$$anon$1.windowOpened(Window.scala:80)
...
[error] Caused by: java.lang.ClassNotFoundException: scala.swing.event.WindowOpened
[error] at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
...
[error] java.lang.NoClassDefFoundError: scala/swing/event/WindowActivated
[error] at scala.swing.Window$$anon$1.windowActivated(Window.scala:74)
...
[error] Caused by: java.lang.ClassNotFoundException: scala.swing.event.WindowActivated
[error] at java.net.URLClassLoader.findClass(URLClassLoader.java:382)
...
The application’s main window shows up but is pretty unusable, with every GUI element generating more errors when activated; even trying to close the window via the window manager fails.
What is missing to run this program?