code hint fixes

src/com/jpexs/browsers/cache/chrome/ChromeCache.java

@@ -54,7 +54,11 @@ public class ChromeCache implements CacheImplementation {
 
     public static ChromeCache getInstance() {
         if (instance == null) {
-            instance = new ChromeCache();
+            synchronized (ChromeCache.class) {
+                if (instance == null) {
+                    instance = new ChromeCache();
+                }
+            }
         }
         return instance;
     }

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

@@ -40,7 +40,11 @@ public class FirefoxCache implements CacheImplementation {
 
     public static FirefoxCache getInstance() {
         if (instance == null) {
-            instance = new FirefoxCache();
+            synchronized (FirefoxCache.class) {
+                if (instance == null) {
+                    instance = new FirefoxCache();
+                }
+            }
         }
         return instance;
     }