Fixes edge case (microsoft#158888)

Author: hediet

src/vs/workbench/contrib/mergeEditor/browser/model/mergeEditorModel.ts

@@ -334,6 +334,10 @@ export class MergeEditorModel extends EditorModel {
 		return ModifiedBaseRangeState.conflicting;
 	}
 
+	public has(baseRange: ModifiedBaseRange): boolean {
+		return this.modifiedBaseRangeHandlingStateStores.get().has(baseRange);
+	}
+
 	public isHandled(baseRange: ModifiedBaseRange): IObservable<boolean> {
 		return this.modifiedBaseRangeHandlingStateStores.get().get(baseRange)!;
 	}

src/vs/workbench/contrib/mergeEditor/browser/model/modifiedBaseRange.ts

@@ -48,7 +48,6 @@ export class ModifiedBaseRange {
 	public readonly input1CombinedDiff = DetailedLineRangeMapping.join(this.input1Diffs);
 	public readonly input2CombinedDiff = DetailedLineRangeMapping.join(this.input2Diffs);
 
-
 	constructor(
 		public readonly baseRange: LineRange,
 		public readonly baseTextModel: ITextModel,

src/vs/workbench/contrib/mergeEditor/browser/view/editors/inputCodeEditorView.ts

@@ -131,6 +131,9 @@ export class InputCodeEditorView extends CodeEditorView {
 				className: derived('checkbox classnames', (reader) => {
 					const classNames = [];
 					const active = viewModel.activeModifiedBaseRange.read(reader);
+					if (!model.has(baseRange)) {
+						return ''; // Invalid state, should only be observed temporarily
+					}
 					const isHandled = model.isHandled(baseRange).read(reader);
 					if (isHandled) {
 						classNames.push('handled');