Fix #1436: Do not crash on unknown assembly reference.

ILSpy.BamlDecompiler.Tests/Cases/NamespacePrefix.xaml

@@ -1,4 +1,4 @@
-<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:cc="clr-namespace:ILSpy.BamlDecompiler.Tests.Cases">
+<Grid xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:cc="clr-namespace:ILSpy.BamlDecompiler.Tests.Cases" xmlns:unk="clr-namespace:A;assembly=Unknown">
 	<cc:CustomControl Grid.Row="0" Tag="{}{}{Test}" CustomName="Custom1">
 		<FrameworkElement.Style>
 			<Style />

ILSpy.BamlDecompiler/Xaml/NamespaceMap.cs

@@ -36,11 +36,11 @@ namespace ILSpy.BamlDecompiler.Xaml {
 
 		public NamespaceMap(string prefix, IModule asm, string xmlNs, string clrNs) {
 			XmlnsPrefix = prefix;
-			Assembly = asm ?? throw new ArgumentNullException(nameof(asm));
+			Assembly = asm;
 			XMLNamespace = xmlNs;
 			CLRNamespace = clrNs;
 		}
 
-		public override string ToString() => $"{XmlnsPrefix}:[{Assembly.Name}|{CLRNamespace ?? XMLNamespace}]";
+		public override string ToString() => $"{XmlnsPrefix}:[{Assembly?.Name}|{CLRNamespace ?? XMLNamespace}]";
 	}
 }

ILSpy.BamlDecompiler/XmlnsDictionary.cs

@@ -37,7 +37,7 @@ namespace ILSpy.BamlDecompiler {
 
 		public string LookupXmlns(IModule asm, string clrNs) {
 			foreach (var ns in this) {
-				if (asm.FullAssemblyName == ns.Assembly.FullAssemblyName && ns.CLRNamespace == clrNs)
+				if (asm.FullAssemblyName == ns.Assembly?.FullAssemblyName && ns.CLRNamespace == clrNs)
 					return ns.XMLNamespace;
 			}
 
@@ -103,14 +103,14 @@ namespace ILSpy.BamlDecompiler {
 
 		public string LookupXmlns(IModule asm, string clrNs) {
 			foreach (var map in piMappings) {
-				if (asm.FullAssemblyName == map.Value.Assembly.FullAssemblyName && map.Value.CLRNamespace == clrNs)
+				if (asm.FullAssemblyName == map.Value.Assembly?.FullAssemblyName && map.Value.CLRNamespace == clrNs)
 					return map.Key;
 			}
 
 			var scope = CurrentScope;
 			while (scope != null) {
 				foreach (var ns in scope) {
-					if (asm.FullAssemblyName == ns.Assembly.FullAssemblyName && ns.CLRNamespace == clrNs)
+					if (asm.FullAssemblyName == ns.Assembly?.FullAssemblyName && ns.CLRNamespace == clrNs)
 						return ns.XMLNamespace;
 				}