How to reproduce:
1. New scala project
2. Create some code, eg:
object App extends Application {
println("foo")
}
3. Set breakpoint (here at line 2)
4. Debug as - Scalla Application - Use configuration specific settings - Scaa Application (new debugger) Launcher - Ok
5. Window: This kind of launch is configured to open the Debug perspective... - Yes
6. Select row in view Variables [IMPORTANT: don't do anything else]

Result:
In lower part of Variables view you can see stacktrace:

exception while invoking toString(): null
scala.tools.eclipse.debug.model.ScalaObjectReference.jdiInvokeMethod(ScalaValue.scala:172)
scala.tools.eclipse.debug.model.HasMethodInvocation$$anonfun$invokeMethod$2.apply(ScalaDebugElement.scala:141)
scala.tools.eclipse.debug.model.HasMethodInvocation$$anonfun$invokeMethod$2.apply(ScalaDebugElement.scala:135)
scala.util.control.Exception$Catch.apply(Exception.scala:102)
scala.tools.eclipse.debug.model.HasMethodInvocation$class.invokeMethod(ScalaDebugElement.scala:135)
scala.tools.eclipse.debug.model.ScalaObjectReference.invokeMethod(ScalaValue.scala:137)
scala.tools.eclipse.debug.model.ScalaDebugModelPresentation$.computeDetail(ScalaDebugModelPresentation.scala:55)
scala.tools.eclipse.debug.model.ScalaDebugModelPresentation$.computeDetail(ScalaDebugModelPresentation.scala:31)
scala.tools.eclipse.debug.model.ScalaDebugModelPresentation$$anon$1.run(ScalaDebugModelPresentation.scala:88)
org.eclipse.core.internal.jobs.Worker.run(Worker.java:53)


Expected result:
result of method toString currently selected object

Workaround:
1. Click on any (even current) row in stacktrace in Debug view
2. Click again on row in Variables view
3. Now lower part of Variables view show expected value

Result of investigation:
Probably variable thread is null in following method invocation:
protected[model] def jdiInvokeMethod(method: Method, thread: ScalaThread, args: Value*): Value = thread.invokeMethod(underlying, method, args:_*) //ScalaValue.scala:172

There is need to initialize
@volatile var currentThread: ScalaThread = null //ScalaDebugger.scala:49
with correct value when switching to Debug perspective or when displaing Debug view.