Reproduction steps:
1. Open the "bug.zip" project.
2. Open the "Test" scene.
3. Play the scene.
4. Notice that texture is being drawn on a scene view toolbar.

Reproduced on: 5.5.0a5, 5.4.0f3, 5.3.5p8, 5.3.5f1, 5.2.5f1

By design:
The reason for this is that we want to be able to blit to a null RenderTexture and therefor cannot do a check or a warning.
If we check the documentation: https://docs.unity3d.com/ScriptReference/Graphics.Blit.html
dest The destination RenderTexture. Set this to null to blit directly to screen.

Doing this inside the RenderObject() also means the user will need to take into account offsets for menus etc.