log failure to find cell/output

Author: minrk

lib/matplotlib/backends/web_backend/nbagg_mpl.js

@@ -44,6 +44,10 @@ mpl.mpl_figure_comm = function(comm, msg) {
 
     fig.parent_element = element.get(0);
     fig.cell_info = mpl.find_output_cell("<div id='" + id + "'></div>");
+    if (!fig.cell_info) {
+        console.error("Failed to find cell for figure", id, fig);
+        return;
+    }
 
     var output_index = fig.cell_info[2]
     var cell = fig.cell_info[0];