fix(gui): resolve NPE on project save (#1463)(PR #1464)

* fix(gui): NullPointerException on project save

* chore(gui): ensure MainWindow.project is never null
* ensure `files` in `ProjectData` in not null

Co-authored-by: Skylot <skylot@gmail.com>

jadx-gui/src/main/java/jadx/gui/settings/data/ProjectData.java

@@ -4,13 +4,14 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 
 import jadx.api.data.impl.JadxCodeData;
 
 public class ProjectData {
 
 	private int projectVersion = 1;
-	private List<Path> files;
+	private List<Path> files = new ArrayList<>();
 	private List<String[]> treeExpansions = new ArrayList<>();
 	private JadxCodeData codeData = new JadxCodeData();
 	private List<TabViewState> openTabs = Collections.emptyList();
@@ -21,7 +22,7 @@ public List<Path> getFiles() {
 	}
 
 	public void setFiles(List<Path> files) {
-		this.files = files;
+		this.files = Objects.requireNonNull(files);
 	}
 
 	public List<String[]> getTreeExpansions() {

jadx-gui/src/main/java/jadx/gui/ui/MainWindow.java

@@ -76,6 +76,7 @@
 import javax.swing.tree.TreeSelectionModel;
 
 import org.fife.ui.rsyntaxtextarea.Theme;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
@@ -178,7 +179,8 @@ public class MainWindow extends JFrame {
 	private final transient JadxSettings settings;
 	private final transient CacheObject cacheObject;
 	private final transient BackgroundExecutor backgroundExecutor;
-	private transient JadxProject project;
+	@NotNull
+	private transient JadxProject project = new JadxProject();
 	private transient Action newProjectAction;
 	private transient Action saveProjectAction;
 
@@ -516,7 +518,7 @@ private List<Path> openProject(Path path) {
 		return jadxProject.getFilePaths();
 	}
 
-	public void updateProject(JadxProject jadxProject) {
+	public void updateProject(@NotNull JadxProject jadxProject) {
 		jadxProject.setSettings(settings);
 		jadxProject.setMainWindow(this);
 		this.project = jadxProject;