title: flutter how to check if Map is null?
body: <p>map defined as follow:</p>
  <pre><code>Map&lt;String,dynamic&gt; allAvailableTime;
  </code></pre>
  <p>the map should be empty when user initialize the page:</p>
  <p>I would like to perform null check before getting data from the map however these are not working...</p>
  <p>check 1:</p>
  <pre><code>allAvailableTime == null ?
  </code></pre>
  <p>error:</p>
  <pre><code>type 'String' is not a subtype of type 'Widget'
  </code></pre>
  <p>check 2:</p>
  <pre><code>allAvailableTime == {} ?
  </code></pre>
  <p>the check should return <code>True</code>, but it returns <code>False</code> instead ..</p>
  <p>check 3:</p>
  <pre><code>allAvailableTime.isEmpty ?
  </code></pre>
  <p>error 3:</p>
  <pre><code>The getter 'isEmpty' was called on null.
  Receiver: null
  Tried calling: isEmpty
  </code></pre>