Problem description:

Error message appears in Pie chart. After changing widget in Data set, submitting configuration and reopening pie chart configuration form.

Steps to reproduce:

1. Create a host with an item, that has type numeric (unsigned) or numeric (float)
2. Create a new dashboard and open it in Edit mode
3. Create Item history widget using the previously created item:
   
4. Create Item navigator widget with default config
5. Create Pie chart widget
   1. Delete default empty data set 
   2. Add new data set -> Item list
   3. Click Add widget -> Item history
      
   4. Submit widget configuration form
6. Click Edit Pie chart configuration
7. Click on Item history in Data set
   
8. In pop up window choose Item navigator
9. Submit widget configuration form
10. Click Edit Pie chart configuration

Result:
Error message appears:

Undefined array key 0 [zabbix.php:17 → require_once() → ZBase->run() → ZBase->processRequest() → ZBase->processResponseFinal() → CView->getOutput() → include() → getDatasetTab() → Widgets\PieChart\Includes\CWidgetFieldDataSetView->getView() → Widgets\PieChart\Includes\CWidgetFieldDataSetView->getDataSetLayout() → CConfigFile->\{closure}() in conf/zabbix.conf.php:89] 

Expected

Error should not appear