title: Failed assertion: line 320 pos 12: 'width > 0.0': is not true for simple sidebar in Flutter
body: <p>I created a sidebar to my app to show some icons:</p>
  <pre><code>Widget build(BuildContext context) {
      return Container(
        color: widget.backgroundColor,
        //width: double.infinity,
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: [
            Container(
                child: FittedBox(
                    child: Image(image: AssetImage('my_icon.png')))),
            FittedBox(child: Image(image: AssetImage('my_st.png'))),
            FittedBox(child: Image(image: AssetImage('my_cart_2.png'))),
            FittedBox(child: Image(image: AssetImage('my_info_2.png'))),
            FittedBox(child: Image(image: AssetImage('my_rotate_2.png')))
          ],
        ),
      );
    }
  </code></pre>
  <p>Even though it renders perfectly, I get this error:</p>
  <pre><code>════════ Exception caught by rendering library ═════════════════════════════════════════════════════
  The following assertion was thrown during performLayout():
  'package:flutter/src/rendering/box.dart': Failed assertion: line 320 pos 12: 'width &gt; 0.0': is not true.
  
  
  Either the assertion indicates an error in the framework itself, or we should provide substantially more information in this error message to help you determine and fix the underlying cause.
  In either case, please report this assertion by filing a bug on GitHub:
    https://github.com/flutter/flutter/issues/new?template=BUG.md
  
  The relevant error-causing widget was: 
    FittedBox file:///home/user/AndroidStudioProjects/flutter_app/lib/sidebar.dart:23:22
  When the exception was thrown, this was the stack: 
  #2      BoxConstraints.constrainSizeAndAttemptToPreserveAspectRatio (package:flutter/src/rendering/box.dart:320:12)
  #3      RenderFittedBox.performLayout (package:flutter/src/rendering/proxy_box.dart:2433:30)
  #4      RenderObject.layout (package:flutter/src/rendering/object.dart:1777:7)
  #5      RenderFlex.performLayout (package:flutter/src/rendering/flex.dart:768:15)
  #6      RenderObject.layout (package:flutter/src/rendering/object.dart:1777:7)
  </code></pre>
  <p>Where the error happens in the first <code>FittedBox</code>. If I remove that, I get the error on the second <code>FittedBox</code>.</p>
  <p>I thought it had something to do with the <code>Image</code> not having width, but setting <code>width</code> to them (just to test) didn't work.</p>
  <p>Even though I get errors, the sidebar renders correctly.</p>