Fix email submission on ios

[brianroper]

[VERBOSE-2:dart_error.cc(16)] Unhandled exception:
PlatformException(Error, Error while launching (null), 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 EstimateFormScreenState._postEstimateRequest (package:ultrasound_solutions/views/estimate_form_screen.dart:129:5)
#4 EstimateFormScreenState._neverSatisfied.. (package:ultrasound_solutions/views/estimate_form_screen.dart:172:32)
#5 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:494:14)
#6 _InkResponseState.build. (package:flutter/src/material/ink_well.dart:549:30)
#7 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:102:24)
#8 TapGestureRecognizer._checkUp (package:flutt<…>