Reproduction steps:(Gif in the Edit)
1) Open attached project.
2) Select any sprite in Assets window, hit Sprite Editor in Inspector.
3) Go into playmode, then exit playmode.

Expected result: Sprite Editor window contents stay unchanged.
Actual result: Sprite Editor window clears up.

Note: Upon selecting any other sprite - it will get loaded into Sprite Editor, but clicking on Sprite Editor button again(Since old sprite is still selected) won't reopen/reload Sprite Editor for that sprite.
Note2: Going into playmode again will actually bring Sprite Editors contents back until you go out of play mode.

Reproduced with: 5.3.4f1, 5.4.0b11
Does not reproduce with: 5.2.4f1