test: kill in fly hinting event

spec/unit/readerui_spec.lua

@@ -34,4 +34,11 @@ describe("Readerui module", function()
         readerui:closeDocument()
         assert(readerui.document == nil)
     end)
+    it("should not reset running_instance by mistake", function()
+        assert.is.truthy(readerui.document)
+        ReaderUI:new{
+            document = DocumentRegistry:openDocument(sample_epub)
+        }:onClose()
+        assert.is.truthy(readerui.document)
+    end)
 end)