Theme parser: class cast exception

When loading themes from xml class cast exceptions can occur:

Caused by: java.lang.ClassCastException: org.nuxeo.theme.fragments.GenericFragment cannot be cast to org.nuxeo.theme.formats.styles.Style
at org.nuxeo.theme.themes.ThemeManager.getNamedStyles(ThemeManager.java:989)
at org.nuxeo.theme.themes.ThemeParser.loadTheme(ThemeParser.java:262)
at org.nuxeo.theme.themes.ThemeParser.registerThemeFromInputStream(ThemeParser.java:173)
at org.nuxeo.theme.themes.ThemeParser.registerTheme(ThemeParser.java:108)
at org.nuxeo.theme.themes.ThemeManager.loadTheme(ThemeManager.java:1098)
at org.nuxeo.theme.themes.ThemeManager.loadTheme(ThemeManager.java:1118)
at org.nuxeo.theme.themes.ThemeManager.deleteTheme(ThemeManager.java:1183)
at org.nuxeo.theme.themes.ThemeManager.uncustomizeTheme(ThemeManager.java:234)
at org.nuxeo.theme.editor.Editor.uncustomizeTheme(Editor.java:548)
at org.nuxeo.theme.editor.Main.uncustomizeTheme(Main.java:757)