Update app/controllers/posts_controller.rb

I found that creating a New Draft, and then immediately deleting it causes the error:

 NoMethodError in PostsController#destroy
undefined method `destroy' for nil:NilClass
Rails.root: C:/Users/Doug/Repositories/obtvse
Application Trace | Framework Trace | Full Trace
app/controllers/posts_controller.rb:94:in `destroy'

This is resolved by first checking "if @post.present?" before calling "@post.destroy".