Throw on null assembly in TryGetType for bad plugin assembly (#1717)

Author: SimonCropp

src/Verify.Tests/PluginConventionTests.cs

@@ -22,6 +22,10 @@ public void TryGetType()
     public void GetTypeName(string assemblyName, string expectedTypeName) =>
         Assert.Equal(expectedTypeName, VerifierSettings.GetTypeName(assemblyName));
 
+    [Fact]
+    public void TryGetTypeNullAssembly() =>
+        Assert.Throws<Exception>(() => VerifierSettings.TryGetType("Verify.BadAssemblyName.dll", out _));
+
     [Fact]
     public void InvokeInitialize()
     {

src/Verify/VerifierSettings_PluginConvention.cs

@@ -51,8 +51,13 @@ internal static bool TryGetType(string file, [NotNullWhen(true)] out Type? type)
             return false;
         }
 #pragma warning disable CS0618
-        var assembly = Assembly.LoadWithPartialName(assemblyName)!;
+        var assembly = Assembly.LoadWithPartialName(assemblyName);
 #pragma warning restore CS0618
+        if (assembly == null)
+        {
+            throw new($"Could not load assembly '{assemblyName}'.");
+        }
+
         var typeName = GetTypeName(assemblyName);
         type = assembly.GetType(typeName);
         return type != null;