Validate widgets cast in dashboard JSON (#1197)

The data structure may not be complete when StateFunc is called, so we
need to assert that the cast works properly.

Closes #1196

Author: therve

datadog/resource_datadog_dashboard_json.go

@@ -57,7 +57,9 @@ func resourceDatadogDashboardJSON() *schema.Resource {
 						delete(attrMap, f)
 					}
 					// Remove every widget id too
-					deleteWidgetID(attrMap["widgets"].([]interface{}))
+					if widgets, ok := attrMap["widgets"].([]interface{}); ok {
+						deleteWidgetID(widgets)
+					}
 					res, _ := structure.FlattenJsonToString(attrMap)
 					return res
 				},