[SalesForce] Return type of an Apex action method must be a PageReference. Found: visualforce.el.VisualforceArrayList

I have the following code which gives me below error.

Return type of an Apex action method must be a PageReference. Found: visualforce.el.VisualforceArrayList

The VF Page I developed.

<apex:page controller="ApexRestController">
    <apex:form >
        <apex:pageBlock >
            <apex:commandButton reRender="table1" value="Get Case Details" action="{!getCaseDetailsById}"/>
        </apex:pageBlock>

        <apex:pageBlock id="table1">
            <b>Output Contact Details </b>
            <apex:pageBlockTable value="{!caList}" var="ca">
                <apex:column headerValue="CaseNumber" value="{!ca.CaseNumber}" />
                <apex:column headerValue="Subject" value="{!ca.Subject}" />
                <apex:column headerValue="Status" value="{!ca.Status}" />
            </apex:pageBlockTable>
        </apex:pageBlock>

    </apex:form>
</apex:page>

And Controller class

public class ApexRestController {
    public List<Case> caList {get; set;}

    public static String getAccessToken(){
        HttpRequest req = new HttpRequest();
        req.setMethod('POST');
        req.setHeader('Content-Type','application/x-www-form-urlencoded');
        req.setEndpoint('https://ap5.salesforce.com/services/oauth2/token');

        String CLIENT_ID = 'XX';
        String CLIENT_SECRET = 'XX';
        String USERNAME = 'XX';
        String PASSWORD = 'XX';

        req.setBody('grant_type=password' + '&client_id='+CLIENT_ID + 
            '&client_secret='+CLIENT_SECRET + '&username='+USERNAME + '&password='+PASSWORD);

        Http http = new Http();
        HTTPResponse res = http.send(req);

        String access_token = res.getBody();
        return access_token;
    } 

    public List<Case> getCaseDetailsById(){
        Http http = new Http();
        HttpRequest req = new HttpRequest();
        req.setEndpoint('https://ap5.salesforce.com/services/apexrest/Cases/XX');
        req.setMethod('GET');
        req.setHeader('Authorization', 'Bearer '+getAccessToken());

        HTTPResponse res = http.send(req);
        String json = res.getBody();

        Case ca = (Case) System.JSON.deserialize(json, Case.class);
        caList = new List<Case>();
        caList.add(ca);
        return caList;
    }
}