Misleading error message if not mounted

[davidmorgan]

Describe the bug

Calling Provider.of<not nullable T> when not mounted triggers a codepath that attempts to throw an error but throws a less good error instead; here:

    if (inheritedElement == null && null is! T) {
      throw ProviderNotFoundException(T, context.widget.runtimeType);
    }

which throws accessing context.widget.

Expected behavior

A better error message could be provided by checking context.isMounted before trying to access widget.