Fix App crash caused by arcgis deinit (#106)

Co-authored-by: QLF3985 <stefan.bergmair@external.t-systems.com>

Author: sbergmair

arcgis_map_sdk_ios/ios/arcgis_map_sdk_ios/Sources/arcgis_map_sdk_ios/ArcgisMapView.swift

@@ -654,7 +654,7 @@ class ArcgisMapView: NSObject, FlutterPlatformView {
             centerChannel.setStreamHandler(nil)
             methodChan.setMethodCallHandler(nil)
         } else {
-            DispatchQueue.main.async {
+            DispatchQueue.main.sync {
                 zoomChannel.setStreamHandler(nil)
                 centerChannel.setStreamHandler(nil)
                 methodChan.setMethodCallHandler(nil)