Add guard against null WebView handler

[mbarta]

Use a safe-call when obtaining a Message from webView.handler and return false if the handler is null. This prevents a potential NPE by aborting the focus href request when the WebView has no handler available.

webView.handler returns null when the WebView has been detached from its window (no longer has a Looper/Handler).

Reported in Sentry with this stacktrace:

java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Message android.os.Handler.obtainMessage()' on a null object reference
    at dev.hotwire.core.turbo.webview.HotwireWebChromeClient.onCreateWindow(SourceFile:1)
    at WV.wm.addNewContents(chromium-SystemWebViewGoogle6432.aab-stable-755913303:39)
    at android.os.MessageQueue.nativePollOnce(MessageQueue.java)
    at android.os.MessageQueue.nextLegacy(MessageQueue.java:950)
    at android.os.MessageQueue.next(MessageQueue.java:1062)
    at android.os.Looper.loopOnce(Looper.java:221)
    at android.os.Looper.loop(Looper.java:363)
    at android.app.ActivityThread.main(ActivityThread.java:10060)
    at java.lang.reflect.Method.invoke(Method.java)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:632)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:975)

org.chromium.base.JniAndroid$UncaughtExceptionException: Native stack trace:
#00 pc 0x00000000066fb53f /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#01 pc 0x0000000001a62163 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#02 pc 0x0000000004b6b033 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#03 pc 0x0000000005be0ff7 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#04 pc 0x0000000005ad2a97 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#05 pc 0x00000000055cd99f /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#06 pc 0x0000000001b65ffb /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#07 pc 0x0000000001b643bf /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#08 pc 0x0000000001b658df /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#09 pc 0x00000000043cc32b /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#10 pc 0x00000000043cc0a7 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#11 pc 0x0000000003753e73 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#12 pc 0x00000000034d7583 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#13 pc 0x00000000034d74fb /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#14 pc 0x00000000039103f7 /data/app/~~4SXFAPhQp5mwiotQKtLUFA==/com.google.android.webview-g0taa4kn34kCmZsBCz3uIA==/base.apk (offset 0xdb8000)
#15 pc 0x0000000000003f9f /system/lib64/libutils.so
#16 pc 0x0000000000003a5b /system/lib64/libutils.so
#17 pc 0x00000000000ef3a3 /system/lib64/libandroid_runtime.so

    at org.chromium.base.JniAndroid.handleException(chromium-SystemWebViewGoogle6432.aab-stable-755913303:21)
    at android.os.MessageQueue.nativePollOnce(MessageQueue.java)
    at android.os.MessageQueue.nextLegacy(MessageQueue.java:950)
    at android.os.MessageQueue.next(MessageQueue.java:1062)
    at android.os.Looper.loopOnce(Looper.java:221)
    at android.os.Looper.loop(Looper.java:363)
    at android.app.ActivityThread.main(ActivityThread.java:10060)
    at java.lang.reflect.Method.invoke(Method.java)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:632)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:975)

[jayohms]

Looks good, thanks.