Fixed NRE in FileExceptionlessLog

The active test run was aborted. Reason: Test host process crashed : Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at Exceptionless.Logging.FileExceptionlessLog.Flush() in /Users/runner/work/Exceptionless.Net/Exceptionless.Net/src/Exceptionless/Logging/FileExceptionlessLog.cs:line 167
   at Exceptionless.Logging.FileExceptionlessLog.OnFlushTimer(Object state) in /Users/runner/work/Exceptionless.Net/Exceptionless.Net/src/Exceptionless/Logging/FileExceptionlessLog.cs:line 260
   at System.Threading.TimerQueueTimer.<>c.<.cctor>b__27_0(Object state)

Author: niemyjski

src/Exceptionless/Logging/FileExceptionlessLog.cs

@@ -138,7 +138,7 @@ public void Flush() {
                 _isFlushing = true;
 
                 Run.WithRetries(() => {
-                    if (!_flushMutex.WaitOne(TimeSpan.FromSeconds(5)))
+                    if (_flushMutex == null || !_flushMutex.WaitOne(TimeSpan.FromSeconds(5)))
                         return;
 
                     hasFlushLock = true;
@@ -163,8 +163,11 @@ public void Flush() {
             } catch (Exception ex) {
                 System.Diagnostics.Trace.WriteLine("Exceptionless: Error flushing log contents to disk: {0}", ex.ToString());
             } finally {
-                if (hasFlushLock)
-                    _flushMutex.ReleaseMutex();
+                if (hasFlushLock) {
+                    // Ensure the mutex hasn't been disposed.
+                    _flushMutex?.ReleaseMutex();
+                }
+
                 _isFlushing = false;
             }
         }
@@ -293,7 +296,7 @@ protected string GetLastLinesFromFile(int lines = 100) {
                     if (lineCount != lines)
                         continue;
 
-                    var returnBuffer = new byte[fs.Value.Length - fs.Value.Position];
+                    byte[] returnBuffer = new byte[fs.Value.Length - fs.Value.Position];
                     fs.Value.Read(returnBuffer, 0, returnBuffer.Length);
 
                     return Encoding.ASCII.GetString(returnBuffer);