Canvas already in the scene on creation

I recently started Unity, and when I went to the Window tab, I saw this thing called LookDev. I decided to use it, and it told me to drag a prefab. I took my Canvas prefab, and not only did it appear in the LookDev, it also appeared in the scene. Even when I closed the LookDev, it was still there. When I clicked the Canvas, nothing happened. Also, when I drag the real prefab Canvas over and move something, fore example an image, the fake canvas would be in the same position and the real one’s position would change, but the image wouldn’t show for the real prefab. So I decided to open a new scene, and guess what I saw: this

As you can see, I only have the MainCamera and the Directional Light, but the Canvas was still there. How does that happen, and how can I fix it?

Never mind, I just closed Unity and opened it again. That made the whole canvas gone.