Fix NPE when no look and feel is configured #3005

Tests

• ✅ key.core.proof_references:test [no report files] (89 succeeded)
• ✅ key.core.symbolic_execution:test [no report files] (361 succeeded, 39 skipped)
• ✅ key.core.testgen:test [no report files] (13 succeeded)
• ✅ key.core:test [no report files] (2873 succeeded, 104 skipped)
• ✅ key.core:testProveRules [no report files] (196 succeeded)
• ❌ key.core:testRunAllFunProofs [no report files] (46 succeeded, 1 failed)
• ✅ key.core:testRunAllInfProofs [no report files] (30 succeeded)
• ✅ key.removegenerics:test [no report files] (45 succeeded)
• ✅ key.ui:test [no report files] (4 succeeded)
• ✅ key.util:test [no report files] (153 succeeded)
• ✅ keyext.exploration:test [no report files] (3 succeeded)
• ✅ recoder:test [no report files] (14 succeeded, 15 skipped)