fix(gui): resolve NPE and fix code style in BreakpointManager

jadx-gui/src/main/java/jadx/gui/device/debugger/BreakpointManager.java

 package jadx.gui.device.debugger;
 
+import java.io.Reader;
 import java.lang.reflect.Type;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.AbstractMap.SimpleEntry;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -14,6 +16,8 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Objects;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -28,52 +32,39 @@ import jadx.gui.treemodel.JClass;
 public class BreakpointManager {
 	private static final Logger LOG = LoggerFactory.getLogger(BreakpointManager.class);
 
-	private static Gson gson = null;
+	private static final Gson GSON = new GsonBuilder().setPrettyPrinting().create();
 	private static final Type TYPE_TOKEN = new TypeToken<Map<String, List<FileBreakpoint>>>() {
 	}.getType();
 
-	private static Map<String, List<FileBreakpoint>> bpm;
-	private static Path savePath;
+	private static @NotNull Map<String, List<FileBreakpoint>> bpm = Collections.emptyMap();
+	private static @Nullable Path savePath;
 	private static DebugController debugController;
 	private static Map<String, Entry<ClassNode, Listener>> listeners = Collections.emptyMap(); // class full name as key
 
 	public static void saveAndExit() {
-		if (bpm != null) {
-			if (bpm.size() == 0 && !Files.exists(savePath)) {
-				return; // user didn't do anything with breakpoint so don't output breakpoint file.
-			}
-			sync();
-			bpm = null;
-			savePath = null;
-			listeners = Collections.emptyMap();
-		}
+		sync();
+		bpm = Collections.emptyMap();
+		savePath = null;
+		listeners = Collections.emptyMap();
 	}
 
-	public static void init(Path dirPath) {
-		if (gson == null) {
-			gson = new GsonBuilder()
-					.setPrettyPrinting()
-					.create();
-		}
-		savePath = dirPath.resolve("breakpoints.json");
+	public static void init(@Nullable Path baseDir) {
+		Path saveDir = baseDir != null ? baseDir : Paths.get(".");
+		savePath = saveDir.resolve("breakpoints.json"); // TODO: move into project file or same dir as project file
 		if (Files.exists(savePath)) {
-			try {
-				byte[] bytes = Files.readAllBytes(savePath);
-				bpm = gson.fromJson(new String(bytes, StandardCharsets.UTF_8), TYPE_TOKEN);
+			try (Reader reader = Files.newBufferedReader(savePath, StandardCharsets.UTF_8)) {
+				bpm = GSON.fromJson(reader, TYPE_TOKEN);
 			} catch (Exception e) {
 				LOG.error("Failed to read breakpoints config: {}", savePath, e);
 			}
 		}
-		if (bpm == null) {
-			bpm = Collections.emptyMap();
-		}
 	}
 
 	/**
 	 * @param listener When breakpoint is failed to set during debugging, this listener will be called.
 	 */
 	public static void addListener(JClass topCls, Listener listener) {
-		if (listeners == Collections.EMPTY_MAP) {
+		if (listeners.isEmpty()) {
 			listeners = new HashMap<>();
 		}
 		listeners.put(DbgUtils.getRawFullName(topCls),
@@ -149,8 +140,15 @@ public class BreakpointManager {
 	}
 
 	private static void sync() {
+		if (savePath == null) {
+			return;
+		}
+		if (bpm.isEmpty() && !Files.exists(savePath)) {
+			// user didn't do anything with breakpoint so don't output breakpoint file.
+			return;
+		}
 		try {
-			Files.write(savePath, gson.toJson(bpm).getBytes(StandardCharsets.UTF_8));
+			Files.write(savePath, GSON.toJson(bpm).getBytes(StandardCharsets.UTF_8));
 		} catch (Exception e) {
 			LOG.error("Failed to write breakpoints config: {}", savePath, e);
 		}