[commands] C++ CommandPtr: Prevent null initialization (#5991)

wpilibNewCommands/src/main/native/cpp/frc2/command/CommandPtr.cpp

@@ -22,6 +22,11 @@
 
 using namespace frc2;
 
+CommandPtr::CommandPtr(std::unique_ptr<Command>&& command)
+    : m_ptr(std::move(command)) {
+  AssertValid();
+}
+
 void CommandPtr::AssertValid() const {
   if (!m_ptr) {
     throw FRC_MakeError(frc::err::CommandIllegalUse,

wpilibNewCommands/src/main/native/include/frc2/command/CommandPtr.h

@@ -27,8 +27,7 @@ namespace frc2 {
  */
 class CommandPtr final {
  public:
-  explicit CommandPtr(std::unique_ptr<Command>&& command)
-      : m_ptr(std::move(command)) {}
+  explicit CommandPtr(std::unique_ptr<Command>&& command);
 
   template <std::derived_from<Command> T>
   // NOLINTNEXTLINE(bugprone-forwarding-reference-overload)
@@ -39,6 +38,8 @@ class CommandPtr final {
   CommandPtr(CommandPtr&&) = default;
   CommandPtr& operator=(CommandPtr&&) = default;
 
+  explicit CommandPtr(std::nullptr_t) = delete;
+
   /**
    * Decorates this command to run repeatedly, restarting it when it ends, until
    * this command is interrupted. The decorated command can still be canceled.

wpilibNewCommands/src/test/native/cpp/frc2/command/CommandPtrTest.cpp

@@ -34,3 +34,7 @@ TEST_F(CommandPtrTest, MovedFrom) {
 
   EXPECT_EQ(1, counter);
 }
+
+TEST_F(CommandPtrTest, NullInitialization) {
+  EXPECT_THROW(CommandPtr{std::unique_ptr<Command>{}}, frc::RuntimeError);
+}