title: Editing column raise `Cannot read property '$sum' of undefined`, Webix DataTable and Firebase
body: <p>I am trying to use Webix DataTable in order to edit records on Firebase and calculate the sum of two records with Math operation.</p>
  
  <p>This is a part of my datatable:</p>
  
  <pre><code>view:"datatable",
  id:"clientTable", 
  select:true, 
  multiselect:true,
  editable:true, 
  editaction:"click",
  math: true,
  footer:true,
  columns:[
      { id:"index", header:"#", sort:"int", adjust:"data"},
      { id:"date", header:"Fecha", sort:"date", editor:"date", fillspace:true, adjust:"data", format:webix.Date.dateToStr("%d/%m/%y"), adjust:"data"},
      { id:"title", header:[ "Producto",{content:"textFilter"}], sort:"string", editor:"text", fillspace:true },
      { id:"bill", header: "Fractura", sort:"int", editor:"text", fillspace:true, text:"0"},
      { id:"amount", header: "Importe", sort:"int", editor:"text", fillspace:true, text:"0"},
      { id:"paid", header: "Pagado", sort:"int", editor:"text", fillspace:true, text:"0"}, 
      { id:"sum",  header:"Suma", math:"[$r,amount] - [$r,paid]"}],               //, cssFormat: mark_sum , footer:{content:"summColumn"}
  
  rules:{
  
      "title": webix.rules.isNotEmpty,
  
  
      "bill": webix.rules.isNotEmpty,
      "amount": webix.rules.isNotEmpty,
      "paid": webix.rules.isNotEmpty,
      "sum": webix.rules.isNotEmpty,
  
  
      "bill": webix.rules.isNumber,
      "amount": webix.rules.isNumber,
      "paid": webix.rules.isNumber,
      "sum": webix.rules.isNumber,
  
      "bill": function(value){ return value &gt; 0 },
      "amount": function(value){ return value &gt; 0 },
      "paid": function(value){ return value &gt;= 0 },
      "sum": function(value){ return value &gt;= 0 }
  
  
  },
  </code></pre>
  
  <p>Everything is working well until i try to edit <code>amount</code> or <code>paid</code> which are calculated in <code>sum</code> column. it's seems like i have a race condition while editing with the <code>Math</code> function.</p>
  
  <p>The exception is:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property '$sum' of undefined
      at h.jn (webix.js:1103)
      at h.jn (webix.js:1104)
      at h.gn (webix.js:1102)
      at webix.DataStore.&lt;anonymous&gt; (webix.js:13)
      at webix.DataStore.callEvent (webix.js:23)
      at webix.DataStore.updateItem (webix.js:524)
      at h.updateItem (webix.js:545)
      at h.ri (webix.js:790)
      at h.&lt;anonymous&gt; (webix.js:788)
      at h.si (webix.js:1115)
  </code></pre>
  
  <p>Is there any work around to make the sum calculate only after editing? or avoid this sum to be calculated on undefined properties?</p>
  
  <p>I've tried to place default values to those columns but it's not helping at all.</p>
  
  <p>Thank you.</p>