title: Error when generating PDF document having an image integrated
body: <p>When I generate a PDF from Orbeon that will get an image (with this url: <code>oxf:/forms/orbeon/dmv-14/images/user_32.png</code>) and add it to the PDF, il works fine.</p>
  
  <p>But when orbeon is embedded in a web application, the PDF document is not generated and we have the following error in the log. Do you have an idea how to resolve this?</p>
  
  <pre><code>An Error has Occurred                                                                                                 submission URL scheme not supported: null                                                                             Application Call Stack
  </code></pre>
  
  <p>and then in the stacktrace :</p>
  
  <pre><code>Exception: org.orbeon.oxf.common.OXFException                                                                         |--|
  |org.orbeon.oxf.util.Connection                     |connect                       |Connection.scala              | 199|
  |----------------------------------------------------------------------------------------------------------------------|
  |Exception: org.orbeon.oxf.common.ValidationException                                                                  |
  |----------------------------------------------------------------------------------------------------------------------|
  |org.orbeon.oxf.util.Connection                     |connect                       |Connection.scala              | 202|
  |org.orbeon.oxf.processor.pdf.PDFTemplateProcessor  |image$lzycompute$1            |PDFTemplateProcessor.scala    | 350|
  |org.orbeon.oxf.processor.pdf.PDFTemplateProcessor  |org$orbeon$oxf$processor$pdf$P|PDFTemplateProcessor.scala    | 317|
  |sor.pdf.PDFTemplateProcessor$$anonfun$handleImage$1|apply                         |PDFTemplateProcessor.scala    | 371|
  |sor.pdf.PDFTemplateProcessor$$anonfun$handleImage$1|apply                         |PDFTemplateProcessor.scala    | 368|
  |scala.Option                                       |foreach                       |Option.scala                  | 257|
  </code></pre>
  
  <h2>Edit:</h2>
  
  <p>@ebruchez
  Below are more details about the problem: 
  - a screenshot of the form which includes an image and one text :<a href="https://i.stack.imgur.com/GICpc.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GICpc.png" alt="enter image description here"></a>
  - a scrennshot of the PDF generated after click on "PDF" using PDF Template. This works fine with orbeon not embedded :
  <a href="https://i.stack.imgur.com/FV3AV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FV3AV.png" alt="enter image description here"></a></p>
  
  <p>When Orbeon is embedded in a web application, the PDF Template is not generated and these are  screenshots of the form and the PDF template.<a href="https://i.stack.imgur.com/O8pNK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/O8pNK.png" alt="enter image description here"></a><a href="https://i.stack.imgur.com/ZzfqQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ZzfqQ.png" alt="enter image description here"></a>
  This is the screenshot of Mapping PDF :<a href="https://i.stack.imgur.com/heULN.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/heULN.png" alt="enter image description here"></a></p>
  
  <p>NOTE : I used the PDF with the processor XHTMLToPDF (without Template) , it works normally.
  Thanks in advance.</p>