[Fix rubocop#9606] Fix an error for Layout/IndentationConsistency

Fixes rubocop#9606.

This PR fixes the following error for `Layout/IndentationConsistency`
when using access modifier at the top level.

```console
% cat example.rb
public

def foo
end

% bundle exec rubocop --only Layout/IndentationConsistency example.rb -d
(snip)

Scanning /Users/koic/src/github.com/koic/rubocop-issues/9606/example.rb
An error occurred while Layout/IndentationConsistency cop was inspecting
/Users/koic/src/github.com/koic/rubocop-issues/9606/example.rb:1:0.
undefined method `source_range' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:165:in
`base_column_for_normal_style'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:181:in
`check_normal_style'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:174:in
`check'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/layout/indentation_consistency.rb:129:in
`on_begin'
```

changelog/fix_error_for_layout_indentation_consistency.md

@@ -0,0 +1 @@
+* [#9606](https://github.com/rubocop/rubocop/issues/9606): Fix an error for `Layout/IndentationConsistency` when using access modifier at the top level. ([@koic][])

lib/rubocop/cop/layout/indentation_consistency.rb

@@ -162,8 +162,10 @@ def base_column_for_normal_style(node)
           # to the level of the module (see `AccessModifierIndentation` cop) we
           # return nil so that `check_alignment` will derive the correct
           # indentation from the first child that is not an access modifier.
-          module_indent = display_column(node.parent.source_range)
           access_modifier_indent = display_column(first_child.source_range)
+          return access_modifier_indent unless node.parent
+
+          module_indent = display_column(node.parent.source_range)
           access_modifier_indent if access_modifier_indent > module_indent
         end
 

spec/rubocop/cop/layout/indentation_consistency_spec.rb

@@ -9,6 +9,33 @@
         "#{}"
       RUBY
     end
+
+    it 'accepts when using access modifier at the top level' do
+      expect_no_offenses(<<~'RUBY')
+        public
+
+        def foo
+        end
+      RUBY
+    end
+
+    it 'registers and corrects an offense when using access modifier and dedented method definition ' \
+       'at the top level' do
+      expect_offense(<<~'RUBY')
+        public
+
+          def foo
+          ^^^^^^^ Inconsistent indentation detected.
+          end
+      RUBY
+
+      expect_correction(<<~'RUBY')
+        public
+
+        def foo
+        end
+      RUBY
+    end
   end
 
   context 'with if statement' do