Avoid crashing in ExplicitOuter on missing references

Author: dwijnand

compiler/src/dotty/tools/dotc/transform/ExplicitOuter.scala

@@ -10,6 +10,7 @@ import core.Phases._
 import core.Types._
 import core.Flags._
 import core.Decorators._
+import core.Scopes.*
 import core.StdNames.nme
 import core.Names._
 import core.NameOps._
@@ -49,7 +50,7 @@ class ExplicitOuter extends MiniPhase with InfoTransformer { thisPhase =>
   /** Add outer accessors if a class always needs an outer pointer */
   override def transformInfo(tp: Type, sym: Symbol)(using Context): Type = tp match {
     case tp @ ClassInfo(_, cls, _, decls, _) if needsOuterAlways(cls) =>
-      val newDecls = decls.cloneScope
+      val newDecls = if decls == EmptyScope then newScope else decls.cloneScope
       newOuterAccessors(cls).foreach(newDecls.enter)
       tp.derivedClassInfo(decls = newDecls)
     case _ =>