test: fix readerui instance reset test

frontend/apps/reader/readerui.lua

@@ -390,6 +390,10 @@ function ReaderUI:doShowReader(file)
     running_instance = reader
 end
 
+function ReaderUI:_getRunningInstance()
+    return running_instance
+end
+
 function ReaderUI:unlockDocumentWithPassword(document, try_again)
     DEBUG("show input password dialog")
     self.password_dialog = InputDialog:new{

spec/unit/readerui_spec.lua

@@ -35,10 +35,13 @@ describe("Readerui module", function()
         assert(readerui.document == nil)
     end)
     it("should not reset running_instance by mistake", function()
-        assert.is.truthy(readerui.document)
+        ReaderUI:doShowReader(sample_epub)
+        local new_readerui = ReaderUI:_getRunningInstance()
+        assert.is.truthy(new_readerui.document)
         ReaderUI:new{
             document = DocumentRegistry:openDocument(sample_epub)
         }:onClose()
-        assert.is.truthy(readerui.document)
+        assert.is.truthy(new_readerui.document)
+        new_readerui:onClose()
     end)
 end)