This principle states that if a practice is considered consistent with the guidelines of medical professionals, there isn't no evidence of negligence or wrongdoing. is or isnt whihc fit better in this context

In this context, the word "isn't" fits better.

The correct sentence would be: "This principle states that if a practice is considered consistent with the guidelines of medical professionals, there isn't any evidence of negligence or wrongdoing."

In this context, the word "isn't" fits better.