Fixes a null pointer exception reported by the PChecker (#713)

Author: ankushdesai

Src/PChecker/CheckerCore/Coverage/ActivityCoverageReporter.cs

@@ -53,7 +53,10 @@ public void EmitCoverageReport(string coverageFile)
         {
             using (var writer = new StreamWriter(coverageFile))
             {
-                WriteCoverageText(writer);
+                if (CoverageInfo.CoverageGraph != null)
+                {
+                    WriteCoverageText(writer);
+                }
             }
         }
 

Src/PCompiler/PCommandLine/CommandLine.cs

@@ -125,7 +125,7 @@ private static void PrintException(Exception ex)
             lock (ConsoleLock)
             {
                 Error.Report($"[Internal Error]:\n {ex.Message}\n<Please report to the P team or create an issue on GitHub, Thanks!>");
-                Error.Report("[PTool] unhandled exception: {0}: {1}", ex.GetType().ToString(), ex.Message);
+                Error.Report("[PTool] unhandled exception: {0}: {1}\n Stack Trace: {2}", ex.GetType().ToString(), ex.Message, ex.StackTrace);
             }
         }