Steps to reproduce:

1. Create dashboard and add as much widgets as necessary to fulfill it. 
2. Try to add one more widget.
3. Observe warning message in widget configuration dialog: "Cannot add widget: not enough free space on the dashboard.".
4. Observe that "Add" button is still active

Result:
"Add" button is enabled.
Expected:
"Add" should be disabled like in Zabbix 4.4.