Fix crash when problemMatcher has empty pattern array (microsoft#256612)

* Initial plan

* Fix empty pattern array crash - add guard clauses

Co-authored-by: anthonykim1 <62267334+anthonykim1@users.noreply.github.com>

* Address review feedback: Add undefined checks to array validation

Co-authored-by: anthonykim1 <62267334+anthonykim1@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: anthonykim1 <62267334+anthonykim1@users.noreply.github.com>

Author: Copilot

src/vs/workbench/contrib/tasks/common/problemMatcher.ts

@@ -987,6 +987,10 @@ export class ProblemPatternParser extends Parser {
 			}
 			result.push(pattern);
 		}
+		if (!result || result.length === 0) {
+			this.error(localize('ProblemPatternParser.problemPattern.emptyPattern', 'The problem pattern is invalid. It must contain at least one pattern.'));
+			return null;
+		}
 		if (result[0].kind === undefined) {
 			result[0].kind = ProblemLocationKind.Location;
 		}
@@ -1042,6 +1046,10 @@ export class ProblemPatternParser extends Parser {
 	}
 
 	private validateProblemPattern(values: IProblemPattern[]): boolean {
+		if (!values || values.length === 0) {
+			this.error(localize('ProblemPatternParser.problemPattern.emptyPattern', 'The problem pattern is invalid. It must contain at least one pattern.'));
+			return false;
+		}
 		let file: boolean = false, message: boolean = false, location: boolean = false, line: boolean = false;
 		const locationKind = (values[0].kind === undefined) ? ProblemLocationKind.Location : values[0].kind;
 

src/vs/workbench/contrib/tasks/test/common/problemMatcher.test.ts

@@ -256,5 +256,13 @@ suite('ProblemPatternParser', () => {
 			assert.strictEqual(ValidationState.Error, reporter.state);
 			assert(reporter.hasMessage('The problem pattern is invalid. It must have at least have a file and a message.'));
 		});
+
+		test('empty pattern array should be handled gracefully', () => {
+			const problemPattern: matchers.Config.MultiLineProblemPattern = [];
+			const parsed = parser.parse(problemPattern);
+			assert.strictEqual(null, parsed);
+			assert.strictEqual(ValidationState.Error, reporter.state);
+			assert(reporter.hasMessage('The problem pattern is invalid. It must contain at least one pattern.'));
+		});
 	});
 });