commandcontext: ensure we init IEnvironment before SessionManager

Ensure we have an instance of IEnvironment before we pass it to the
SessionManager contructor.

Also add some null guards to catch this problem earlier in the future.

Author: mjcheetham

src/shared/Core/CommandContext.cs

@@ -118,8 +118,8 @@ public CommandContext()
             else if (PlatformUtils.IsMacOS())
             {
                 FileSystem        = new MacOSFileSystem();
-                SessionManager    = new MacOSSessionManager(Environment, FileSystem);
                 Environment       = new MacOSEnvironment(FileSystem);
+                SessionManager    = new MacOSSessionManager(Environment, FileSystem);
                 ProcessManager    = new ProcessManager(Trace2);
                 Terminal          = new MacOSTerminal(Trace);
                 string gitPath    = GetGitPath(Environment, FileSystem, Trace);
@@ -134,8 +134,8 @@ public CommandContext()
             else if (PlatformUtils.IsLinux())
             {
                 FileSystem        = new LinuxFileSystem();
-                SessionManager    = new LinuxSessionManager(Environment, FileSystem);
                 Environment       = new PosixEnvironment(FileSystem);
+                SessionManager    = new LinuxSessionManager(Environment, FileSystem);
                 ProcessManager    = new ProcessManager(Trace2);
                 Terminal          = new LinuxTerminal(Trace);
                 string gitPath    = GetGitPath(Environment, FileSystem, Trace);

src/shared/Core/ISessionManager.cs

@@ -22,6 +22,9 @@ public abstract class SessionManager : ISessionManager
 
         protected SessionManager(IEnvironment env, IFileSystem fs)
         {
+            EnsureArgument.NotNull(env, nameof(env));
+            EnsureArgument.NotNull(fs, nameof(fs));
+
             Environment = env;
             FileSystem = fs;
         }