Fix emergency call feature on IOS

[brianroper]

PlatformException(Error, Error while launching tel://18007734582, null)
#0 StandardMethodCodec.decodeEnvelope (package:flutter/src/services/message_codecs.dart:547:7)
#1 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:279:18)

#2 launch (package:url_launcher/url_launcher.dart:47:19)
#3 HomeScreenState._buildHomeList.. (package:ultrasound_solutions/views/home_screen.dart:52:17)
#4 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:494:14)
#5 _InkResponseState.build. (package:flutter/src/material/ink_well.dart:549:30)
#6 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
#7 TapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:161:9)
#8 TapGestureRecognizer.acceptGesture (package:flutter/src/gestures/tap.dart:123:7)
#9 G<…>