Swift, print issue when use ? and ! variables : <p>I got access error('fatal error: unexpectedly found nil while unwrapping an Optional value') when print variable p02. According to the error message, I still cannot understand the reason.</p>

<pre><code>var p02:Person! = nil
var p03:Person? = nil

if (p02==nil)
{
    print("p02 is nil")
}
if (p03==nil)
{
    print("p03 is nil")
}
print(p03)
print(p02)
</code></pre>

<p>Anyone know why, 
Thanks</p>
