Fix crash parsing an empty multi-text that contains an empty span (#1338

)

- This bug caused a crash importing into FieldWorks

SIL.Lift.Tests/Parsing/ParserTests.cs

@@ -10,7 +10,7 @@
 using SIL.Lift.Parsing;
 using SIL.Lift.Tests.Properties;
 using SIL.Lift.Validation;
-using SIL.TestUtilities;
+using Unit = NUnit.Framework;
 using Has = NMock2.Has;
 using Is = NMock2.Is;
 
@@ -160,6 +160,23 @@ public void MultiTextWithNestedSpan()
 			Assert.AreEqual(0, subspan.Spans.Count);
 		}
 
+		[Test]
+		public void MultiTextWithEmptyNestedSpan()
+		{
+			_doc.LoadXml("<foobar><form lang='x'><text><span lang='y'></span></text></form></foobar>");
+			_parser.ParsingWarning += (sender, args) => { Assert.Fail("Could not parse empty span in an empty multitext.");};
+			var t = _parser.ReadMultiText(_doc.FirstChild);
+			var tx = t["x"];
+			Assert.That(tx, Unit.Is.Not.Null);
+			Assert.That(tx.Text, Unit.Is.Null);
+			Assert.That(tx.Spans.Count, Unit.Is.EqualTo(1));
+			var span = tx.Spans[0];
+			Assert.That(span, Unit.Is.Not.Null);
+			Assert.That(span.Class, Unit.Is.Null);
+			Assert.That(span.Lang, Unit.Is.EqualTo("y"));
+			Assert.That(span.Length, Unit.Is.EqualTo(0));
+		}
+
 		[Test]
 		public void FirstValueOfSimpleMultiText()
 		{

SIL.Lift/Parsing/LiftMultiText.cs

@@ -408,7 +408,7 @@ public LiftSpan AddSpan(string key, string lang, string style, string href, int
 				alternative = new LiftString();
 				this[key] = alternative;
 			}
-			int start = alternative.Text.Length;
+			int start = alternative.Text?.Length ?? 0;
 			if (lang == key)
 				lang = null;
 			var span = new LiftSpan(start, length, lang, style, href);