Fixed possible NPE.

core/src/main/java/com/github/weisj/darklaf/components/border/DarkBorders.java

@@ -27,13 +27,16 @@ import javax.swing.*;
 import javax.swing.border.Border;
 import java.awt.*;
 import java.lang.ref.WeakReference;
+import java.util.Collections;
 import java.util.Map;
 import java.util.WeakHashMap;
 
 public final class DarkBorders {
 
-    public static Map<WeakLineBorder, WeakReference<WeakLineBorder>> lineBorderMap = new WeakHashMap<>();
-    private static Map<WeakLineBorder, WeakReference<WeakLineBorder>> lineWidgetBorderMap = new WeakHashMap<>();
+    private static Map<WeakLineBorder, WeakReference<WeakLineBorder>> lineBorderMap =
+        Collections.synchronizedMap(new WeakHashMap<>());
+    private static Map<WeakLineBorder, WeakReference<WeakLineBorder>> lineWidgetBorderMap =
+        Collections.synchronizedMap(new WeakHashMap<>());
 
 
     public static Border createLineBorder(final int top, final int left, final int bottom, final int right) {
@@ -45,14 +48,16 @@ public final class DarkBorders {
                                        final Map<WeakLineBorder, WeakReference<WeakLineBorder>> map, final String key) {
         WeakLineBorder border = new WeakLineBorder(top, left, bottom, right);
         if (map.containsKey(border)) {
-            return map.get(border).get();
-        } else {
+            WeakReference<WeakLineBorder> ref = map.get(border);
+            if (ref != null) {
+                WeakLineBorder b = ref.get();
+                if (b != null) return b;
+            }
+        }
         border.setColor(UIManager.getColor(key));
         map.put(border, new WeakReference<>(border));
         return border;
     }
-    }
-
 
     public static Border createWidgetLineBorder(final int top, final int left, final int bottom, final int right) {
         return createBorder(top, left, bottom, right, lineWidgetBorderMap, "borderSecondary");