HLE: Update ProcessResult in ProcessLoader.cs

Some games/applications may crash with a specific error:

00:00:00.690 |E| Application : Unhandled exception caught: System.Collections.Generic.KeyNotFoundException: The given key '0' was not present in the dictionary.

(or a different key)

However, this message is misleading and not indicative of the actual error (it masks it).

As an example, the error above was seen when attempting to load a mod onto Yo-kai Watch 1 (base game + update worked). However, after patching the ProcessResult, the resulting error was more revealing:

00:00:00.716 |E| Application : Unhandled exception caught: LibHac.Common.HorizonResultException: ResultFsInvalidCharacter (2002-6004): Error creating LocalFileSystem. at LibHac.FsSystem.LocalFileSystem..ctor(String rootPath)

(and so forth)

The actual error was due to an incorrect folder name in the Mods folder.

src/Ryujinx.HLE/Loaders/Processes/ProcessLoader.cs

@@ -24,7 +24,16 @@ namespace Ryujinx.HLE.Loaders.Processes
 
         private ulong _latestPid;
 
-        public ProcessResult ActiveApplication => _processesByPid[_latestPid];
+        public ProcessResult ActiveApplication
+        {
+            get
+            {
+                if (_latestPid != 0 && _processesByPid.TryGetValue(_latestPid, out ProcessResult process))
+                    return process;
+
+                return null;
+            }
+        }
 
         public ProcessLoader(Switch device)
         {