To reproduce:
1. Open attached Unity project "RTReplace.zip"
2. Open "0" scene
3. In the Hierarchy, navigate to Canvas --> RawImage
4. In RawImage Inspector, change Texture from RT1 to none
5. In the ChangeRT script of the Camera object, uncomment line saying "Camera.main.targetTexture = null;"
6. Go into Play mode

Expected result: RawImage should stay active and visible
Actual result: RawImage disappears instantly after starting Play mode and it is invisible in scene view too, despite that there is no place in the code that could trigger this behaviour (see attached gif "bug.gif")

Workaround: Making Camera.targetTexture wait a few frames until setting it to null does not reproduce the issue (see attached gif "nobug.gif")

Reproduced on: 5.6.3p3, 2017.1.1p2, 2017.2.0f1, 2017.3.0b1