title: TextFormField is not working. Here is the code and problem below
body: <pre><code>Scaffold(
      body:Stack(
          children:[
              Positioned(
                  top:120,
                  child:Container(
                      child:Column(
                          children:[
                              TextFormField(
                                  validator: (value) {
                                      if (value == null || value.isEmpty) {
                                          return 'Please enter some text';
                                      }
                                      return null;
                                  },
                              ),
                          ]
                      )
                  )
              )   
          ]
      )
  );
  </code></pre>
  <p>This message shows:</p>
  <blockquote>
  <p>The following RenderObject was being processed when the exception was fired: _RenderDecoration#03a2b<br />
  relayoutBoundary=up20 NEEDS-LAYOUT NEEDS-PAINT NEEDS-COMPOSITING-BITS-UPDATE:
  needs compositing<br />
  creator: _Decorator ← InputDecorator ← AnimatedBuilder ← Listener ← RawGestureDetector ← TextSelectionGestureDetector ← Semantics ← AnimatedBuilder ← IgnorePointer ← _RawMouseRegion ← MouseRegion ← TextField ← ⋯<br />
    parentData:  (can use size)<br />
    constraints: BoxConstraints(unconstrained)<br />
    size: MISSING</p>
  </blockquote>