Fatal Exception: NSInvalidArgumentException Crash on iOS (returned bug)

Issue occurs on: iOS
Plugin version: 0.3.10

Crash repeating on iPhoneX iOS 13.3.0
```
Fatal Exception: NSInvalidArgumentException
0 CoreFoundation 0x1ad6dea48 __exceptionPreprocess
1 libobjc.A.dylib 0x1ad405fa4 objc_exception_throw
2 CoreFoundation 0x1ad5e25a8 -[NSOrderedSet initWithSet:copyItems:]
3 CoreFoundation 0x1ad6e2af4 ___forwarding___
4 CoreFoundation 0x1ad6e4a7c _CF_forwarding_prep_0
5 CoreFoundation 0x1ad6a4008 _CFURLCreateWithURLString
6 Foundation 0x1ad99cb94 +[NSURL(NSURL) URLWithString:relativeToURL:]
7 Runner 0x10433b274 -[FlutterWebviewPlugin navigate:] + 180 (FlutterWebviewPlugin.m:180)
8 Runner 0x10433acc0 -[FlutterWebviewPlugin initWebview:] + 144 (FlutterWebviewPlugin.m:144)
9 Runner 0x10433a0f8 -[FlutterWebviewPlugin handleMethodCall:result:] + 51 (FlutterWebviewPlugin.m:51)
10 Flutter 0x104b2532c (Missing)
11 Flutter 0x104ac58fc (Missing)
12 Flutter 0x104b168dc (Missing)
13 Flutter 0x104ad3628 (Missing)
14 Flutter 0x104ad5f80 (Missing)
15 CoreFoundation 0x1ad65d03c __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__
16 CoreFoundation 0x1ad65cd78 __CFRunLoopDoTimer
17 CoreFoundation 0x1ad65c448 __CFRunLoopDoTimers
18 CoreFoundation 0x1ad657584 __CFRunLoopRun
19 CoreFoundation 0x1ad656adc CFRunLoopRunSpecific
20 GraphicsServices 0x1b75dc328 GSEventRunModal
21 UIKitCore 0x1b1751ae0 UIApplicationMain
22 Runner 0x1042f93e0 main + 6 (AppDelegate.swift:6)
23 libdyld.dylib 0x1ad4e0360 start
```

Inizia ispezionando FlutterWebviewPlugin.m in navigate: alla riga 180, quindi segui come initWebview: alla riga 144 e handleMethodCall:result: alla riga 51 vi arrivano. Usa i dettagli forniti relativi al crash e alla riproduzione su iOS 13.3.0 per indagare sul percorso di costruzione dell’URL. Il lavoro è completato quando la navigazione segnalata non genera più una NSInvalidArgumentException.

Scritto dal modello di indicizzazione a partire dal testo della issue.