Merge pull request #622 from Quick/7.x-pr508

[7.x] Provide fallback messages to prevent crashes when main run loop is busy

Author: ikesyo

Sources/Nimble/Matchers/Async.swift

@@ -23,14 +23,18 @@ private func async<T>(style: ExpectationStyle, predicate: Predicate<T>, timeout:
         }
         switch result {
         case .completed: return lastPredicateResult!
-        case .timedOut: return PredicateResult(status: .fail, message: lastPredicateResult!.message)
+        case .timedOut:
+            let message = lastPredicateResult?.message ?? .fail("timed out before returning a value")
+            return PredicateResult(status: .fail, message: message)
         case let .errorThrown(error):
             return PredicateResult(status: .fail, message: .fail("unexpected error thrown: <\(error)>"))
         case let .raisedException(exception):
             return PredicateResult(status: .fail, message: .fail("unexpected exception raised: \(exception)"))
         case .blockedRunLoop:
             // swiftlint:disable:next line_length
-            return PredicateResult(status: .fail, message: lastPredicateResult!.message.appended(message: " (timed out, but main thread was unresponsive)."))
+            let message = lastPredicateResult?.message.appended(message: " (timed out, but main run loop was unresponsive).") ??
+                .fail("main run loop was unresponsive")
+            return PredicateResult(status: .fail, message: message)
         case .incomplete:
             internalError("Reached .incomplete state for \(fnName)(...).")
         }