Don't call .toString() on potentially null CharSequence

We're building an exception message string, but by explicitly invoking
.toString() we're accidentally triggering an NPE rather than the typed
exception we want to throw.  Build the string in a way that will be
safe and sensical even if the CharSequence is null.

Bug 29009255

Change-Id: I1813260f0b36fd44506b8327f997dd20c2d6c8bf