Validate default exchange selection against active exchanges

[reddink]

was noticing the following error when trying to setup a couple of different coins on a new device
It was preventing the addition of some coins on device

FATAL EXCEPTION: main
Process: com.brentpanther.bitcoinwidget.debug, PID: 17457
java.lang.IllegalArgumentException: No enum constant com.brentpanther.bitcoinwidget.exchange.Exchange.COINDESK
	at java.lang.Enum.valueOf(Enum.java:300)
	at com.brentpanther.bitcoinwidget.exchange.Exchange.valueOf(Unknown Source:2)
	at com.brentpanther.bitcoinwidget.ui.selection.CoinSelectionViewModel$createWidget$2.invokeSuspend(CoinSelectionViewModel.kt:49)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:34)
	at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:100)
	at kotlinx.coroutines.internal.LimitedDispatcher$Worker.run(LimitedDispatcher.kt:124)
	at kotlinx.coroutines.scheduling.TaskImpl.run(Tasks.kt:89)
	at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:586)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:820)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:717)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:704)
	Suppressed: kotlinx.coroutines.internal.DiagnosticCoroutineContextException: [androidx.compose.ui.platform.MotionDurationScaleImpl@c0b2818, androidx.compose.runtime.BroadcastFrameClock@1879271, StandaloneCoroutine{Cancelling}@9bdf856, AndroidUiDispatcher@1120d7]

The following change should allow for fallback to CoinGecko if enum is missing.
Its going to:

• look up all exchange names that support the given currency (e.g. "USD"). If none are found, immediately return "COINGECKO" as the default.
• get the list of all exchange names currently defined in the Exchange enum
• find the first exchange from the JSON data that also exists in the enum, and if no match is found, fall back to COINGECKO.

[hwki]

@reddink Do you have an example of how to get this error to occur? I'm having trouble reproducing it.

[reddink]

in my case, i was using both reddcoin and vertcoin.
in my testing,
Add widget
select current price -> add
search redd or vert -> go
select the result

it is at this point the app will close for both these items ( i didnt check others)
selecting other coins bitcoin, litecoin and doge works as expected

from what i could tell, there was a attempt to reuse the last created widgets exchange,
if the coin was not avaulable on that exchange (eg binance) it would fall through to getdefaultexchange
returns exchanges?.get(0) — the first exchange in the list for that currency.
Based on the available JSON and ordering, for reddcoin and vertcoin that was COINDESK (not COINGECKO).

COINDESK is in the available json, but it is not in the exchange enum

[hwki]

Ok, that makes sense now. The real reason this is coming up is that when I removed exchanges like CoinDesk, I neglected to remove them from the json file as well. I'll merge this, in case it happens again but ideally it shouldn't.