Handle suspended units in InteractiveDriver

Author: nicolasstucki

compiler/src/dotty/tools/dotc/interactive/InteractiveDriver.scala

@@ -155,7 +155,7 @@ class InteractiveDriver(val settings: List[String]) extends Driver {
 
       run.compileSources(List(source))
       run.printSummary()
-      val unit = ctx.run.units.head
+      val unit = if ctx.run.units.nonEmpty then ctx.run.units.head else ctx.run.suspendedUnits.head
       val t = unit.tpdTree
       cleanup(t)
       myOpenedTrees(uri) = topLevelTrees(t, source)