Problem description: for some prototypes, like for item prototype "

: Disk read rate" on the default setup of Zabbix server host, when creating a Graph prototype the preview shows the following errors:

Steps to reproduce:

1. Have a clean Zabbix installation
2. Open the list of Graph prototypes for "Block devices discovery" LLD on Zabbix server host
3. Press "Create graph prototype" button
4. In the opened form add a name and select prototype "{#DEVNAME}: Disk read rate" in Items field
5. Open "Preview" tab

Result: the above errors are shown in preview tab instead of the graph prototype preview.
Expected: a graph with no data should be shown in preview