fix issue in which projects which have forks cannot be changed from private to public

app/helpers/projects_helper.rb

@@ -148,7 +148,7 @@ module ProjectsHelper
   def can_change_visibility_level?(project, current_user)
     return false unless can?(current_user, :change_visibility_level, project)
 
-    if project.fork_source
+    if project.forked? && project.fork_source
       project.fork_source.visibility_level > Gitlab::VisibilityLevel::PRIVATE
     else
       true