Fix calling polymorphic functions

tests/run/polymorphic-functions.scala

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/init/Objects.scala

@@ -314,7 +314,7 @@ class Objects {
           Result(Bottom, Errors.empty)
 
         case TypeAbs(tp) =>
-          if meth.isEffectivelyFinal then
+          if meth.exists && meth.isEffectivelyFinal then
             if meth.hasSource then
               val isLocal = meth.owner.isClass
               val ddef = meth.defTree.asInstanceOf[DefDef]