appDisplay: Don't assume this._placeholder always exists

appDisplay: Don't assume this._placeholder always exists

Dropping a icon from the dash to the application grid will set this._placeholder
to null. However the AppIcon is still used to represent the application in the
application grid. If we click on it we emit a pressed event. Stop assuming
that this._placeholder is still valid in the callback, use the icon parameter
instead.

This fixes the following error:
```
Gjs-CRITICAL **: 18:22:39.003: JS ERROR: TypeError: this._placeholder is null
_ensurePlaceholder/<@resource:///org/gnome/shell/ui/appDisplay.js:1477:17
vfunc_button_press_event@resource:///org/gnome/shell/ui/appDisplay.js:3121:27
```

Fixes: 6fc93b78bc54763c0876951f2425fcfd96f61f00
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6317
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2606>