Avoid immediate source of crash in TypeErasure for #16368

Fixes #16368 (?)

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -735,9 +735,10 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
             // but potentially re-introduced by ResolveSuper, when we add
             // forwarders to mixin methods.
             // See doc comment for ElimByName for speculation how we could improve this.
-        else
+        else if rt.exists then
           MethodType(Nil, Nil,
             eraseResult(rt.translateFromRepeated(toArray = sourceLanguage.isJava)))
+        else rt
       case tp1: PolyType =>
         eraseResult(tp1.resultType) match
           case rt: MethodType => rt