J-P Nurmi 6321cd7b64 [Linux][FlView] guard against disconnecting a disconnected signal (flutter/engine#35490)
GObject's dispose() method may be called multiple times. Guard against
trying to disconnect the same signal multiple times by clearing the ID
to avoid warnings when closing the window.

```
(bug:74019): GLib-GObject-WARNING **: 11:15:08.697: ../../../gobject/gsignal.c:2731: instance '0x55e1c3ea0200' has no handler with id '255'
```
2022-08-25 13:27:20 -07:00
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