kcms/decoration: Fix crash when preview cannot be generated
m_bridge->createDecoration() can return nullptr. Most codepaths already guard against that, but not all of them
BUG: 456531 (cherry picked from commit 0fef2295)
m_bridge->createDecoration() can return nullptr. Most codepaths already guard against that, but not all of them
BUG: 456531 (cherry picked from commit 0fef2295)