I don't know why this message appears

[rafaznj]

Before, I was using the swing normally, but a while later, this message started to appear.

Someone help me please.

[ptziegler]

This problem has most likely been logged somewhere. Can you post the full stack trace?
If you click on Window -> Show View -> Error Log in the menu bar, this should open a new view. There you simply have to right-click on the error item, select "Copy" and then paste the clipboard content here.

[SimulAsian]

I'm getting the same issue. Here's the full stack trace for mine:
java.lang.NullPointerException: Cannot invoke "org.eclipse.ui.IWorkbenchPage.getActivePart()" because the return value of "org.eclipse.jdt.internal.ui.JavaPlugin.getActivePage()" is null
at org.eclipse.jdt.ui.wizards.NewContainerWizardPage.getInitialJavaElement(NewContainerWizardPage.java:234)
at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
at java.base/java.lang.reflect.Method.invoke(Method.java:580)
at org.eclipse.wb.internal.core.utils.reflect.ReflectionUtils.invokeMethod(ReflectionUtils.java:896)
at org.eclipse.wb.internal.core.utils.reflect.ReflectionUtils.invokeMethodEx(ReflectionUtils.java:875)
at org.eclipse.wb.internal.core.wizards.WizardUtils.getJavaElement(WizardUtils.java:47)
at org.eclipse.wb.internal.core.wizards.WizardUtils.getJavaProject(WizardUtils.java:33)
at org.eclipse.wb.internal.core.wizards.AbstractDesignWizard.getJavaProject(AbstractDesignWizard.java:169)
at org.eclipse.wb.internal.core.wizards.AbstractDesignWizard.finishPage(AbstractDesignWizard.java:87)
at org.eclipse.wb.internal.core.wizards.DesignerNewElementWizard$2.run(DesignerNewElementWizard.java:116)
at org.eclipse.jdt.internal.core.BatchOperation.executeOperation(BatchOperation.java:41)
at org.eclipse.jdt.internal.core.JavaModelOperation.run(JavaModelOperation.java:740)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2451)
at org.eclipse.core.internal.resources.Workspace.run(Workspace.java:2476)
at org.eclipse.jdt.core.JavaCore.run(JavaCore.java:6086)
at org.eclipse.wb.internal.core.wizards.DesignerNewElementWizard$WorkbenchRunnableAdapter.run(DesignerNewElementWizard.java:213)
at org.eclipse.jface.operation.ModalContext$ModalContextThread.run(ModalContext.java:124)

[ptziegler]

@SimulAsian Thank you for the stack trace. It seems like the issue only shows up when trying to create a snippet when no project is selected. Conversely, you can work around this problem by opening the wizard via right-clicking on the project in the Project Explorer.