First of all, note that the warning is generated by your compiler (or static analyzer, or linter), not by your debugger, as you initially wrote.

The warning is telling you that your program possibly might dereference a null pointer. The reason for this warning is that you perform a malloc() and then use the result (the pointer) without checking for NULL values. In this specific code example, malloc() will most likely just return the requested block of memory. On any desktop computer or laptop, there's generally no reason why it would fail to allocate 12 bytes. That's why your application just runs fine and exits successfully. However, if this would be part of a larger application and/or run on a memory-limited system such as an embedded system, malloc() could fail and return NULL. Note that malloc() does not only fail if there is not enough memory available, it could also fail if there is no large enough consecutive block of memory available, due to fragmentation.

According to the C standard, dereferencing a NULL pointer is undefined behavior, meaning that anything could happen. On modern computers it would likely get your application killed (which could lead to data loss or corruption, depending on what the application does). On older computers or embedded systems the problem might be undetected and your application would read from or (worse) write to the address NULL (which is most likely 0, but even that isn't guaranteed by the C standard). This could lead to data corruption, crashes or other unexpected behavior at an arbitrary time after this happened.

Note that the compiler/analyzer/linter doesn't know anything about your application or the platform you will be running it on, and it doesn't make any assumptions about it. It just warns you about this possible problem. It's up to you to determine if this specific warning is relevant for your situation and how to deal with it.

Generally speaking, there are three things you can do about it:

1. If you know for sure that malloc() would never fail (for example, in such a toy example that you would only run on a modern computer with gigabytes of memory) or if you don't care about the results (because the application will be killed by your OS and you don't mind), then there's no need for this warning. Just disable it in your compiler, or ignore the warning message.

2. If you don't expect malloc() to fail, but do want to be informed when it happens, the quick-and-dirty solution is to add assert(v != NULL); after the malloc. Note that this will also exit your application when it happens, but in a slightly more controlled way, and you'll get an error message stating where the problem occurred. I would recommend this for simple hobby projects, where you do not want to spend much time on error handling and corner cases but just want to have some fun programming :-)

3. When there is a realistic change that malloc() would fail and you want a well-defined behavior of your application, you should definitely add code to handle that situation (check for NULL values). If this is the case, you would generally have to do more than just add an if-statement. You would have to think about how the application can continue to work or gracefully shutdown without requiring more memory allocations. And on an embedded system, you would also have to think about things such as memory fragmentation.

The easiest fix for the example code in question is add the NULL-check. This would make the warning go away, and (assuming malloc() would not fail) your program would run still the same.

int main(void) {
    uint32_t *v = malloc(3 * sizeof(uint32_t));
    if (v != NULL) {
        v[0] = 12;
        v[1] = 59; 
        v[2] = 83; 
        twice_three(v); 
        free(v); 
    }
    return 0; 
}

You've not checked that the memory allocation was successful, and malloc() could (but probably won't) return NULL, and then you'd be dereferencing a null pointer in the loops. So, if you do as your textbook tells you to do and check the return value from malloc(), you can probably avoid the error.

If your textbook didn't say something similar to "Always check the return value from malloc() and its friends", then you should forcibly throw your textbook into a furnace where it can no longer mislead you or anyone else.

Note that a simple Google search using c6011finds Warning C6011 and the description starts:

This warning indicates that your code dereferences a potentially null pointer. If the pointer value is invalid, the result is undefined. To resolve the issue, validate the pointer before use.

The problem lies in your deleteFromList function, with this code:

while (temp->data != n && temp != NULL) {
//...

Here, you are trying to check the value of temp->data before you have verified whether or not temp is NULL. Thus, you will, at some point (when you're at the end of the list, and temp is NULL be dereferencing a null pointer - which ain't good!

Instead, just invert the order of the comparisons:

while (temp != NULL && temp->data != n) {
//...

This way, as soon as temp is NULL, the comparison's result will be fully known (see short circuiting), temp->data will not be evaluated, and the loop will stop running.

These is a warning from the static analyzer. The compiler says that there might be something potentially wrong.

And indeed, in DLL_CreateNode there is a call to malloc that could fail and return a NULL. You check it inside the function, which is fine.

But if malloc fails, you would still return a NULL to the caller in main.c.

NewNode = DLL_CreateNode(i);   //<-- this could return NULL
DLL_AppendNode(&List, NewNode);

In this case, this NULL would be given to DLL_AppendNode as the second parameter.

If there is an existing head node at this point in time, then in DLL_AppendNode at this line:

NewNode->Prev = Tail;

it would give an EXC_BAD_ACCESS exception.

The static analyzer has thus correctly identified a potential problem.

There are several ways to deal with it. One solution would be to check for NULL, print an error message on stderr, and exit the program with an error code.

In MSVC, defining __declspec(noreturn) for abort_function should do it.
For gcc, __attribute__ ((noreturn)) does the same.