CSS dialog disabling

Thirst test case:
1) Open any CSS file.
2) Press button on the panel bar (Open CSS dialog based on the active CSS file)
ASSERT: CSS dialog appears
3) Close CSS dialog
RESULT: Button disappeared from the panel bar

Second test case:
1) Create an empty CSS file
2) Paste any source from another CSS file
RESULT: The only way to call CSS dialog (not a wizard) on existing CSS style class is to call it from context menu. Any other way is blocked. This situation doesn't even change after saving CSS file (Ctrl+S)

Third test case:
1) Open any CSS file with stable CSS style classes (it's better to use attached CSS file)
2) Choose last styel class in a list
3) Open CSS dialog on it
4) Press "Clear" button
5) Press "Ok"
6) Delete the last empty style class from the page (do not save any changes)
7) Try to call CSS dialog
RESULT: The situation is the same as in the second test case.