Repository: holoviz/panel

Issue:
Player Widget does not loop when going forward, only backward

#### ALL software version info
Not sure of the panel version on the linked webpage, assuming the most recent.
On my installation, it's 0.11.1. Firefox 87.0.

#### Description of expected behavior and the observed behavior


Set widget cursor near the end. Set loop policy to loop.
Press play. Expected behavior: loops around to the start. 
What happens: once the last frame is reached, stays there.
I looked here, but found nothing obviously wrong : https://github.com/holoviz/panel/blob/master/panel/models/player.ts


#### Complete, minimal, self-contained example code that reproduces the issue

Go to https://panel.holoviz.org/reference/widgets/Player.html and use the player there.
Or in a jupyter lab notebook:
```python
import panel as pn
pn.extension()
pn.widgets.Player(value=0, start=0, end=5, interval=100,loop_policy='loop')
```

#### Stack traceback and/or browser JavaScript console output

Same on my installation and linked tutorial page.
```
Uncaught Error: panel.models.widgets.Player(1037).value given invalid value: null
    validate https://panel.holoviz.org/reference/widgets/Player.html:461
    _update https://panel.holoviz.org/reference/widgets/Player.html:461
    set_value https://panel.holoviz.org/reference/widgets/Player.html:461
    _setv https://panel.holoviz.org/reference/widgets/Player.html:457
    setv https://panel.holoviz.org/reference/widgets/Player.html:457
    set https://panel.holoviz.org/reference/widgets/Player.html:457
    set_frame https://panel.holoviz.org/reference/widgets/Player.html:1270
    first_frame https://panel.holoviz.org/reference/widgets/Player.html:1270
    anim_step_forward https://panel.holoviz.org/reference/widgets/Player.html:1270
    timer https://panel.holoviz.org/reference/widgets/Player.html:1270
    setInterval handler*play_animation https://panel.holoviz.org/reference/widgets/Player.html:1270
    onclick https://panel.holoviz.org/reference/widgets/Player.html:1270
Player.html:461:2451
    validate https://panel.holoviz.org/reference/widgets/Player.html:461
    _update https://panel.holoviz.org/reference/widgets/Player.html:461
    set_value https://panel.holoviz.org/reference/widgets/Player.html:461
    _setv https://panel.holoviz.org/reference/widgets/Player.html:457
    setv https://panel.holoviz.org/reference/widgets/Player.html:457
    set https://panel.holoviz.org/reference/widgets/Player.html:457
    set_frame https://panel.holoviz.org/reference/widgets/Player.html:1270
    first_frame https://panel.holoviz.org/reference/widgets/Player.html:1270
    anim_step_forward https://panel.holoviz.org/reference/widgets/Player.html:1270
    timer https://panel.holoviz.org/reference/widgets/Player.html:1270
    (Async: setInterval handler)
    play_animation https://panel.holoviz.org/reference/widgets/Player.html:1270
    onclick https://panel.holoviz.org/reference/widgets/Player.html:1270

```
#### Screenshots or screencasts of the bug in action

![image](https://user-images.githubusercontent.com/4441338/113451334-af2c7c00-93f9-11eb-8cdd-777275310a57.png)


#### Leads

![image](https://user-images.githubusercontent.com/4441338/113451759-940e3c00-93fa-11eb-823f-8ff19aaec0a2.png)

Seems model.start is not set on the javascript side.




Discussion:

Resolving patch:
diff --git a/panel/models/widgets.py b/panel/models/widgets.py
--- a/panel/models/widgets.py
+++ b/panel/models/widgets.py
@@ -16,9 +16,9 @@ class Player(Widget):
     The Player widget provides controls to play through a number of frames.
     """
 
-    start = Int(help="Lower bound of the Player slider")
+    start = Int(0, help="Lower bound of the Player slider")
 
-    end = Int(help="Upper bound of the Player slider")
+    end = Int(10, help="Upper bound of the Player slider")
 
     value = Int(0, help="Current value of the player app")
 


Test patch: