do not create default project on app dispose

To avoid exceptions on exit like:
java.lang.RuntimeException: Memory leak detected: 'com.intellij.openapi.vfs.encoding.EncodingManagerImpl@695b8c2c' of class com.intellij.openapi.vfs.encoding.EncodingManagerImpl
See the cause for the corresponding Disposer.register() stacktrace:

platform/platform-impl/src/com/intellij/openapi/project/impl/ProjectManagerImpl.java

@@ -331,6 +331,7 @@ public class ProjectManagerImpl extends ProjectManagerEx implements Disposable {
   public synchronized Project getDefaultProject() {
     LOG.assertTrue(!myDefaultProjectWasDisposed, "Default project has been already disposed!");
     if (myDefaultProject == null) {
+      LOG.assertTrue(!ApplicationManager.getApplication().isDisposeInProgress(), "Application being disposed!");
       ProgressManager.getInstance().executeNonCancelableSection(() -> {
         try {
           myDefaultProject = createProject(null, "", true);

platform/platform-impl/src/com/intellij/openapi/wm/impl/welcomeScreen/FlatWelcomeFrame.java

@@ -759,6 +759,7 @@ public class FlatWelcomeFrame extends JFrame implements IdeFrame, Disposable, Ac
   @Nullable
   @Override
   public Project getProject() {
+    if (ApplicationManager.getApplication().isDisposeInProgress()) return null;
     return ProjectManager.getInstance().getDefaultProject();
   }