Avoid NPE when duplicating symbols

Fixes #10542

Author: odersky

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

@@ -835,8 +835,10 @@ object Symbols {
           info = completer,
           privateWithin = ttmap1.mapOwner(odenot.privateWithin), // since this refers to outer symbols, need not include copies (from->to) in ownermap here.
           annotations = odenot.annotations)
-        copy.registeredCompanion =
-          copy.registeredCompanion.subst(originals, copies)
+        copy.denot match
+          case cd: ClassDenotation =>
+            cd.registeredCompanion = cd.unforcedRegisteredCompanion.subst(originals, copies)
+          case _ =>
       }
 
       copies.foreach(_.ensureCompleted()) // avoid memory leak

tests/pos/i10542.scala

@@ -0,0 +1,29 @@
+package test1:
+
+  trait Foo {
+    inline def foo[A](t: => A): Unit = ()
+  }
+
+  object Bar extends Foo
+
+  object Test {
+    Bar.foo {
+      sealed trait T1
+      case object S1 extends T1
+    }
+  }
+/*
+package test2:
+
+  trait Foo {
+    inline def foo[A](t: => A): Unit = ()
+  }
+
+  object Bar extends Foo
+
+  object Test {
+    Bar.foo {
+      trait T1
+      val array = Array(new T1 {})
+    }
+  }*/