Fix exception (#3158)

app/src/main/java/com/alphawallet/app/service/OpenSeaService.java

@@ -392,6 +392,8 @@ public class OpenSeaService
             ownerOption = "owner_address";
         }
 
+        if (!TextUtils.isEmpty(api))
+        {
             Uri.Builder builder = new Uri.Builder();
             builder.encodedPath(api)
                 .appendQueryParameter(ownerOption, address)
@@ -401,6 +403,9 @@ public class OpenSeaService
             return executeRequest(networkId, builder.build().toString());
         }
 
+        return JsonUtils.EMPTY_RESULT;
+    }
+
     public String fetchAsset(long networkId, String contractAddress, String tokenId)
     {
         String api = "";