Fix ProofTreeView NPE #3070

Tests

• ✅ key.core.testgen:test (13 succeeded)
• ✅ key.core:test (2925 succeeded, 102 skipped)
• ❌ key.core:testProveRules (195 succeeded, 1 failed)
• ❌ key.core:testRunAllFunProofs (46 succeeded, 1 failed)
• ✅ key.core:testRunAllInfProofs (30 succeeded)
• ✅ key.removegenerics:test (45 succeeded)
• ✅ key.ui:test (5 succeeded)
• ✅ key.util:test (132 succeeded)
• ✅ keyext.exploration:test (3 succeeded)
• ✅ recoder:test (14 succeeded, 15 skipped)