fixed Cache viewer when cache directory does not exist (browser not installed)

trunk/src/com/jpexs/browsers/cache/firefox/FirefoxCache.java

@@ -34,6 +34,9 @@ public class FirefoxCache implements CacheImplementation {
     @Override
     public void refresh() {
         File dir = getCacheDirectory();
+        if(dir==null){
+            return;
+        }
         File cacheMapFile = new File(dir, "_CACHE_MAP_");
         try {
             map = new CacheMap(cacheMapFile);
@@ -105,6 +108,9 @@ public class FirefoxCache implements CacheImplementation {
         if (profileDir != null) {
             cacheDir = new File(profileDir, "Cache");
         }
+        if(!cacheDir.exists()){
+            return null;
+        }
         return cacheDir;
     }