debug NPE

Author: felixmulder

compiler/test/dotty/tools/vulpix/SummaryReport.scala

@@ -83,8 +83,10 @@ final class SummaryReport extends SummaryReporting {
   def addStartingMessage(msg: String): Unit =
     startingMessages.append(msg)
 
-  def addCleanup(f: () => Unit): Unit =
+  def addCleanup(f: () => Unit): Unit = {
+    assert(f ne null)
     cleanUps.append(f)
+  }
 
   /** Both echoes the summary to stdout and prints to file */
   def echoSummary(): Unit = {