avoid crashing when not subclassing IGListSectionController #117

Job	Run time
Verify Carthage build lipo binaries	3m 2s
CocoaPods Lint	6m 11s
Verify Carthage build XCFramework	4m 10s
Build Examples and UI tests. (platform=iOS Simulator,name=iPhone 14 Pro,OS=16.4, platform=tvOS Si...	17m 43s
Run Danger	0s
Unit Test iOS (platform=iOS Simulator,name=iPhone 14 Pro)	2m 25s
Unit Test macOS	2m 54s
Verify generate_spm_sources_layout.sh is not broken	1m 46s
Verify SPM build by invoking `xcodebuild` on Package.swift (IGListDiffKit)	1m 34s
Verify SPM build by invoking `xcodebuild` on Package.swift (IGListKit)	1m 40s
Verify SPM build by invoking `xcodebuild` on Package.swift (IGListSwiftKit)	1m 33s
	42m 58s

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