Fixing 2 bugs that happen when fields are removed from table (#551)

caravel/assets/javascripts/explore.js

@@ -237,7 +237,8 @@ function initExploreView() {
   function set_filters() {
     for (var i = 1; i < 10; i++) {
       var eq = px.getParam("flt_eq_" + i);
-      if (eq !== '') {
+      var col = px.getParam("flt_col_" + i);
+      if (eq !== '' && col !== '') {
         add_filter(i);
       }
     }

caravel/forms.py

@@ -52,7 +52,7 @@ class SelectMultipleSortableField(SelectMultipleField):
             d[value] = (value, label, selected)
         if self.data:
             for value in self.data:
-                if value:
+                if value and value in d:
                     yield d.pop(value)
         while d:
             yield d.popitem(last=False)[1]