fix potential NPE in TopScopeObject

Author: wirthi

src/main/java/org/truffleruby/debug/TopScopeObject.java

@@ -227,7 +227,7 @@ void removeMember(String member,
             @Shared("interop") @CachedLibrary(limit = "LIMIT") InteropLibrary interop)
             throws UnsupportedMessageException, UnknownIdentifierException {
         int length = NAMES.length;
-        for (int i = 0; i < length; i++) {
+        for (int i = scopeIndex; i < length; i++) {
             Object scope = this.objects[i];
             if (interop.isMemberRemovable(scope, member)) {
                 interop.removeMember(scope, member);