only create filter provider if download is complete

EDDB/API.cs

@@ -10,6 +10,10 @@ namespace NonaBGS.EDDB {
 
         private string systems_file = null;
 
+        public delegate void DatabaseAvailableDelegate();
+
+        public event DatabaseAvailableDelegate SystemsAvailable;
+
         public string SystemsFile => systems_file;
 
         public string Cache {
@@ -27,13 +31,15 @@ namespace NonaBGS.EDDB {
             client.DownloadDataCompleted += Client_DownloadDataCompleted;
         }
 
-        private void DownloadFile(string url, string file) {
-            client.DownloadFileAsync(new Uri(url), file);
+        private void DownloadFile(string url, string file, DatabaseAvailableDelegate notifier) {
+            client.DownloadFileAsync(new Uri(url), file, notifier);
         }
 
         public void Download(bool force) {
             if (!HaveSystemsFile || force) {
-                DownloadFile(EDDB_SYSTEMS_ARCHIVE, systems_file);
+                DownloadFile(EDDB_SYSTEMS_ARCHIVE, systems_file, SystemsAvailable);
+            } else if (HaveSystemsFile) {
+                SystemsAvailable?.Invoke();
             }
         }
 
@@ -54,6 +60,8 @@ namespace NonaBGS.EDDB {
         }
 
         private void Client_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e) {
+            DatabaseAvailableDelegate notifier = e.UserState as DatabaseAvailableDelegate;
+            notifier?.Invoke();
         }
     }
 }