title: type 'Null' is not a subtype of type 'String' flutter / dart
body: <p>I have this json and i want to show <code>variation</code>'s keys and values (i should extract keys and values because they're not stable).</p>
  <pre><code>{
    &quot;data&quot; : [
       &quot;items&quot;: {
                  &quot;388488&quot;: {
                      &quot;id&quot;: 388488,
                      &quot;name&quot;: &quot;Galaxy Tab A7 10.4 2020&quot;,
                      &quot;qty&quot;: 1,
                      &quot;variations&quot;: {
                          &quot;color&quot;: &quot;green&quot;,
                                     },
                             },
                  &quot;388489&quot;: {
                      &quot;id&quot;: 388489,
                      &quot;name&quot;: &quot;Samsung Galaxy Tab A7 10.4 2020&quot;,
                      &quot;qty&quot;: 1,
                      &quot;variations&quot;: {
                          &quot;color&quot;: &quot;red&quot;,
                          &quot;something&quot;: &quot;somth&quot;
                                     },
                  },
                  &quot;388490&quot;: {
                      &quot;id&quot;: 388490,
                      &quot;name&quot;: &quot; Apple iPad 10.2&quot;,
                      &quot;qty&quot;: 1,
                      &quot;variations&quot;: {
                          &quot;color&quot;: &quot;blue&quot;,
                          &quot;something1&quot;: &quot;somth1&quot;
                                     },
                  }
              }
         ]
   }
  </code></pre>
  <p>when i print variation in this way it is fine and print all variations :</p>
  <pre><code>  itemDetail.variations.forEach((key, value) {
         print(value);
       });
  </code></pre>
  <p>but when i try to show it in a text widget it says variation is <strong>null</strong> :</p>
  <pre><code>                  Text(
                              itemDetail.variations.forEach((key , value){
                                String Description = &quot;$key : $value&quot;;
                                print(Description); // it just print the variation of first item
                                return Description; // I also tried without return, same error
                              }),
                              textDirection: TextDirection.rtl,),
  </code></pre>
  <p><strong>itemDetail</strong> is an object of Item model class
  here it is:</p>
  <pre><code>class Items{
    late final id;
    late final name;
    late final quantity;
    late final variations;
    Items({
     this.id,
     this.name,
     this.quantity,
      this.variations
  });
    
    
    factory Items.fromJson(Map&lt;String , dynamic&gt; Json){
      return Items(
        id: Json[&quot;id&quot;],
        name: Json[&quot;name&quot;],
        quantity: Json[&quot;qty&quot;],
          variations: Json[&quot;variations&quot;] ?? &quot;&quot;
      );
    }
  }
  </code></pre>