Add model test

src/main/java/app/model/Model.java

@@ -15,8 +15,8 @@ import javafx.scene.Scene;
 public class Model {
   private static boolean fileIsSaved;
   private static Optional<Path> activeFilePath;
-  private static Optional<Path> currentProjectPath;
-  private static ProgrammingLanguage currentProgrammingLanguage;
+  private static Optional<Path> activeProjectPath;
+  private static ProgrammingLanguage activeProgrammingLanguage;
   private static String theme;
   private static Scene scene;
   private static SettingsProvider settings;
@@ -26,19 +26,23 @@ public class Model {
   }
 
   public static void setActiveFilePath(Optional<Path> path) {
+    if (path == null)
+      throw new IllegalArgumentException("path can not be null");
     Model.activeFilePath = path;
   }
 
   public static Optional<Path> getProjectPath() {
-    return currentProjectPath;
+    return activeProjectPath;
   }
 
   public static void setProjectPath(Optional<Path> path) {
-    Model.currentProjectPath = path;
+    if (path == null)
+      throw new IllegalArgumentException("path can not be null");
+    Model.activeProjectPath = path;
   }
 
   public static ProgrammingLanguage getLanguage() {
-    return currentProgrammingLanguage;
+    return activeProgrammingLanguage;
   }
 
   public static Scene getScene() {
@@ -58,14 +62,20 @@ public class Model {
   }
 
   public static void setTheme(String theme) {
+    if (theme == null)
+      throw new IllegalArgumentException("theme can not be null");
     Model.theme = theme;
   }
 
   public static void setLanguage(ProgrammingLanguage language) {
-    Model.currentProgrammingLanguage = language;
+    if (language == null)
+      throw new IllegalArgumentException("language can not be null");
+    Model.activeProgrammingLanguage = language;
   }
 
   public static void setScene(Scene scene) {
+    if (scene == null)
+      throw new IllegalArgumentException("scene can not be null");
     Model.scene = scene;
   }
 
@@ -74,6 +84,8 @@ public class Model {
   }
 
   public static void setSettingsProvider(SettingsProvider settings) {
+    if (settings == null)
+      throw new IllegalArgumentException("settings can not be null");
     Model.settings = settings;
   }