In the code above the list is populated using OGNL expression. That is valid expression, however IDE not always has support it since the OGNL version and plugin version may mismatch. You can always exclude it from validation. But it's not the major problem and you could not pay attention to it.
This example uses the same collection and looks fine
<s:select list="#@java.util.LinkedHashMap@{'-1':'All Activity','0':'Posts Only','1':'Trades Only'}"/>