Exceptions from watch methods are swallowed on startup

[willmcgugan]

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from textual.reactive import reactive
from textual.widgets import Footer, Header, Static


class UnhandledExceptionTest(App):
    BINDINGS = [("q", "quit", "Quit")]

    def compose(self) -> ComposeResult:
        yield Header()
        yield MyStatic()
        yield Footer()

    def on_mount(self):
        self.query_one(MyStatic).contents = "Hello, World!"


class MyStatic(Static):
    contents = reactive(None, always_update=True)

    def watch_contents(self, new):
        self.update(new.upper())


if __name__ == "__main__":
    UnhandledExceptionTest().run()

The upper in watch_contents throws an exception because the initial value of the reactive attribute is None. Yes, this would not happen in real life as the type checker warns about it, but it does work to show how the exception is gobbled up.
I just can't figure out why my bigger app will terminate in 0.11 but not 0.12, but that demo code above does gobble the exception and does not terminate in both 0.11 and 0.12.
(at least on my two different machines, a work computer and home computer)

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