title: Flutter 'owner!._debugCurrentBuildTarget == this': is not true. #87
body: <p>I am getting the error when I try to load the pdf from url. It shows the number of pages of pdf but then it crashes.</p>
  <p>Using plugin - <a href="https://pub.dev/packages/advance_pdf_viewer" rel="noreferrer">https://pub.dev/packages/advance_pdf_viewer</a></p>
  <pre><code>════════════════════════════════════════════════════════════════════════════════════════════════════
  I/flutter (28488): ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
  I/flutter (28488): The following assertion was thrown while rebuilding dirty elements:
  I/flutter (28488): 'package:flutter/src/widgets/framework.dart': Failed assertion: line 4371 pos 14:
  I/flutter (28488): 'owner!._debugCurrentBuildTarget == this': is not true.
  I/flutter (28488):
  I/flutter (28488): Either the assertion indicates an error in the framework itself, or we should provide substantially
  I/flutter (28488): more information in this error message to help you determine and fix the underlying cause.
  I/flutter (28488): In either case, please report this assertion by filing a bug on GitHub:
  I/flutter (28488):   https://github.com/flutter/flutter/issues/new?template=BUG.md
  I/flutter (28488):
  I/flutter (28488): The relevant error-causing widget was:
  I/flutter (28488):   PDFViewer
  I/flutter (28488):   
  lib/…/panSignatureScreens/pan_screen.dart:156
  I/flutter (28488):
  I/flutter (28488): When the exception was thrown, this was the stack:
  I/flutter (28488): #2      Element.rebuild.&lt;anonymous closure&gt; 
  package:flutter/…/widgets/framework.dart:4371
  I/flutter (28488): #3      Element.rebuild 
  package:flutter/…/widgets/framework.dart:4374
  I/flutter (28488): #4      BuildOwner.buildScope 
  package:flutter/…/widgets/framework.dart:2777
  I/flutter (28488): #5      WidgetsBinding.drawFrame 
  package:flutter/…/widgets/binding.dart:906
  I/flutter (28488): #6      RendererBinding._handlePersistentFrameCallback 
  package:flutter/…/rendering/binding.dart:309
  I/flutter (28488): #7      SchedulerBinding._invokeFrameCallback 
  package:flutter/…/scheduler/binding.dart:1117
  I/flutter (28488): #8      SchedulerBinding.handleDrawFrame 
  package:flutter/…/scheduler/binding.dart:1055
  I/flutter (28488): #9      SchedulerBinding._handleDrawFrame 
  package:flutter/…/scheduler/binding.dart:971
  I/flutter (28488): #13     _invoke  (dart:ui/hooks.dart:251:10))
  I/flutter (28488): #14     _drawFrame  (dart:ui/hooks.dart:209:3))
  </code></pre>
  <p>my usage - inside a Column(children:[])</p>
  <pre><code>new Expanded(
                    child: Container(
                      child: PDFViewer(
                        document: document,
                      ),
                      decoration: BoxDecoration(
                        border: Border.all(color: Colors.black),
                      ),
                    ),
                  ),
  </code></pre>