title: Unable to enter variable to SliverAppBar title value
body: <p>I have an application. I've added a page to my app and that page will have product information. I was also able to redirect to the page and show the information on that page. I want to put the name of the product in the title value of the <code>SliverAppBar</code>. The product name is written in the <code>productName</code> variable.</p>
  <p>But I can't enter a variable in the <code>SliverAppBar</code>'s <code>title</code> value. When trying to login I get this error:</p>
  <p><a href="https://i.stack.imgur.com/pgLSc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pgLSc.png" alt="enter image description here" /></a></p>
  <pre><code>Evaluation of this constant expression throws an exception.dart(const_eval_throws_exception)
  
  A value of type 'Null' can't be assigned to a parameter of type 'String' in a const constructor.
  Try using a subtype, or removing the keyword 'const'.dartconst_constructor_param_type_mismatch
  
  Avoid using braces in interpolation when not needed.dartunnecessary_brace_in_string_interps
  
  Invalid constant value.
  </code></pre>
  <hr />
  <p>Codes:</p>
  <pre><code>child: CustomScrollView(
    slivers: &lt;Widget&gt;[
      const SliverAppBar(
        pinned: true,
        expandedHeight: 250.0,
        flexibleSpace: FlexibleSpaceBar(
          title: Text(&quot;Product ${productName}&quot;),
        ),
      ),
    // ...
  </code></pre>
  <p>How can I solve this problem? Thanks in advance for your help.</p>