Depends Upon "Text Field" Value the Drop down List has to populate with the related value

[Kumar]

Hi Team,

I have a Two custom Fields one is Text Field (Text SI) and the other one is Drop Down List (Source) here is the thing I have 20 Text field Values we are using while we creating the Tickets and  we have like 20 Values in dropdown List Field  also which is related to Text Field Values.

I need when I select the Particular Value From the Text Field automatically the Dropdown List has to Populate with The related Value. 

Here i have tried one script when i Select the Particular Text Field Value its  populating the dropdown list with "Null" Value. But I need to populate the dropdown List with Value.

Customfield_120805 //Text Field

Customfield_120822 // Select List

if(getFieldById("customfield_120805").getValue().equals("High Severity")){
 getFieldById("customfield_120822").setHidden(false)
 getFieldById("customfield_120822").setRequired(true)
}else{
 getFieldById("customfield_120822").setFormValue(null)
 getFieldById("customfield_120822").setRequired(false)
 getFieldById("customfield_110822").setHidden(true)
}

 I tried insted of "Null" i gave the Value from DropDown List but it didnot work.

So can you please help me with this script 

Thanks,

Kumar