Importing / closed projects throw a lot of errors

Hello everyone,

I noticed that when importing projects, there are a lot of errors like this in the `Error Log` view:

> isGWTProject(project) hasGwtnature exception Project 'CertificateManager' is not open.

It seems to originate from [HERE](https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/v4.2.0/plugins/com.gwtplugins.gwt.eclipse.core/src/com/google/gwt/eclipse/core/nature/GWTNature.java#L53-L57). I guess that the [ResourceChangeListener](https://github.com/gwt-plugins/gwt-eclipse-plugin/blob/v4.2.0/plugins/com.gwtplugins.gdt.eclipse.core/src/com/google/gdt/eclipse/core/reference/ResourceChangeListener.java) might be implemented incorrectly, assuming all projects are opened, which is then triggering the initial issue in `GWTNature#isGwtProject`.

When importing projects, we do that automatically, and initially, they are always closed before being opened. We don't have a special implementation; we rely on the Eclipse API.

Once everything is finished and the projects are all imported (and open), there are no further errors by the GWT plug-in, and everything works as expected. But this is quite annoying nevertheless.

Best,
Tobias

Bắt đầu với GWTNature#isGwtProject trong plugins/com.gwtplugins.gwt.eclipse.core/src/com/google/gwt/eclipse/core/nature/GWTNature.java và ResourceChangeListener trong plugins/com.gwtplugins.gdt.eclipse.core/src/com/google/gdt/eclipse/core/reference/ResourceChangeListener.java. Tái hiện việc import khi các project đang đóng, sau đó theo dõi quá trình xử lý thay đổi tài nguyên; hoàn thành khi việc import các project đã đóng không còn tạo ra các lỗi được báo cáo và việc phát hiện project thông thường vẫn hoạt động.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.