<p>Mapping a non existing ID_KEY to a blank value would show the following error popup when starting iD:</p>
<p><a target="_blank" href="https://user-images.githubusercontent.com/5842757/39259636-37e207ea-48b7-11e8-93a5-5dd25864820e.png"><img src="https://user-images.githubusercontent.com/5842757/39259636-37e207ea-48b7-11e8-93a5-5dd25864820e.png" alt="grafik" style="max-width:100%;"></a></p>
<div class="highlight highlight-source-ruby"><pre>diff <span class="pl-k">--</span>git a<span class="pl-k">/</span>app<span class="pl-k">/</span>views<span class="pl-k">/</span>site<span class="pl-k">/</span><span class="pl-c1">_id</span>.html.erb b<span class="pl-k">/</span>app<span class="pl-k">/</span>views<span class="pl-k">/</span>site<span class="pl-k">/</span><span class="pl-c1">_id</span>.html.erb
index 867b148..57a8967 <span class="pl-c1">100644</span>
<span class="pl-k">---</span> a<span class="pl-k">/</span>app<span class="pl-k">/</span>views<span class="pl-k">/</span>site<span class="pl-k">/</span><span class="pl-c1">_id</span>.html.erb
<span class="pl-k">+++</span> b<span class="pl-k">/</span>app<span class="pl-k">/</span>views<span class="pl-k">/</span>site<span class="pl-k">/</span><span class="pl-c1">_id</span>.html.erb
@@ <span class="pl-k">-</span><span class="pl-c1">1</span>,<span class="pl-c1">7</span> <span class="pl-k">+</span><span class="pl-c1">1</span>,<span class="pl-c1">7</span> @@
 <span class="pl-k"><</span><span class="pl-k">%=</span> javascript_include_tag <span class="pl-s"><span class="pl-pds">"</span>edit/id<span class="pl-pds">"</span></span> <span class="pl-s"><span class="pl-pds">%></span></span>
<span class="pl-s"> </span>
<span class="pl-s"> <div id="map"<span class="pl-pds">></span></span>
<span class="pl-k">-</span>  <span class="pl-k"><</span><span class="pl-k">%</span> data <span class="pl-k">=</span> { <span class="pl-c1">:key</span> => <span class="pl-c1">ID_KEY</span> } <span class="pl-k">-</span><span class="pl-s"><span class="pl-pds">%></span></span>
<span class="pl-s">+  <% data = { :key =<span class="pl-pds">></span></span> <span class="pl-k">defined?</span>(<span class="pl-c1">ID_KEY</span>) <span class="pl-k">?</span> <span class="pl-c1">ID_KEY</span> : <span class="pl-s"><span class="pl-pds">"</span><span class="pl-pds">"</span></span> } <span class="pl-k">-</span><span class="pl-s"><span class="pl-pds">%></span></span>
<span class="pl-s">   <% data[:lat] = @lat if @lat -%<span class="pl-pds">></span></span>
   <span class="pl-k"><</span><span class="pl-k">%</span> data[<span class="pl-c1">:lon</span>] <span class="pl-k">=</span> <span class="pl-smi">@lon</span> <span class="pl-k">if</span> <span class="pl-smi">@lon</span> <span class="pl-k">-</span><span class="pl-s"><span class="pl-pds">%></span></span>
<span class="pl-s">   <% data[:gpx] = trace_data_url(params[:gpx], :format =<span class="pl-pds">></span></span> <span class="pl-c1">:xml</span>) <span class="pl-k">if</span> params[<span class="pl-c1">:gpx</span>] <span class="pl-k">-</span><span class="pl-s"><span class="pl-pds">%></span></span>
<span class="pl-s"></span>
<span class="pl-s"></span></pre></div>

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