crash on iOS and iOS-Sim

[extrawurst]

crashes in the simulator right away and on device on second call:

#1	0x00000001853f1a4c in -[UIApplication _openURL:options:completionHandler:] ()
#2	0x0000000104bd7754 in _$LT$$LP$A$C$B$C$C$RP$$u20$as$u20$objc..message..MessageArguments$GT$::invoke::h723bf370353936c5 [inlined] at /Users/stephan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc-0.2.7/src/message/mod.rs:128
#3	0x0000000104bd7744 in objc::message::platform::send_unverified::h633786a288498e5d [inlined] at /Users/stephan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc-0.2.7/src/message/apple/mod.rs:27
#4	0x0000000104bd7744 in objc::message::send_message::h51c0bf550976ad86 [inlined] at /Users/stephan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/objc-0.2.7/src/message/mod.rs:178
#5	0x0000000104bd7744 in webbrowser::os::open_browser_internal::h265347fd9891411b at /Users/stephan/code/webbrowser-rs/src/ios.rs:34
#6	0x0000000104bd6ee8 in webbrowser::open_browser_with_options::h4f9ae24e53e16fcc [inlined] at /Users/stephan/code/webbrowser-rs/src/lib.rs:331
#7	0x0000000104bd6eb8 in webbrowser::open_browser::h7ea0b174943835c6 [inlined] at /Users/stephan/code/webbrowser-rs/src/lib.rs:298
#8	0x0000000104bd6eb8 in webbrowser::open::h36cf092bc1244f75 at /Users/stephan/code/webbrowser-rs/src/lib.rs:283