Ensure we init IEnvironment before SessionManager (#1167)

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

@@ -119,8 +119,8 @@ 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, Trace2);
                 string gitPath    = GetGitPath(Environment, FileSystem, Trace);
@@ -136,8 +136,8 @@ 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, Trace2);
                 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;
         }