Null check intent before starting "learn more".

It is possible in certain build configurations to have an invalid
link. Instead of crashing, swallow the error and write to logs.

BUG: 28381984
Change-Id: I19f20b96afb42758187707f1b86a5817e8d737a8

src/com/android/settings/fingerprint/FingerprintEnrollIntroduction.java

@@ -183,6 +183,10 @@ public class FingerprintEnrollIntroduction extends FingerprintEnrollBase
         public void onClick(View widget) {
             Context ctx = widget.getContext();
             Intent intent = HelpUtils.getHelpIntent(ctx, getURL(), ctx.getClass().getName());
+            if (intent == null) {
+                Log.w(LearnMoreSpan.TAG, "Null help intent.");
+                return;
+            }
             try {
                 // This needs to be startActivityForResult even though we do not care about the
                 // actual result because the help app needs to know about who invoked it.