Unordered_map iterator iterates to a null value even though I don't put any null values in the map

I am storing a bunch of dummy objects in an unordered_map, and then iterating through to delete them. Somehow, it comes out with a key value pairing where the value is a null pointer, even though I’m pretty sure that is not possible.

Each of the objects that I make just houses a part of a very large string that I construct randomly. I make sure to hand the constructor of this object at least 5 characters of the string.

Here is the code:

Delete Function

bool delete_objects(){
    Serial.println("deleting objects in 'store'");
    for(auto iter = store.begin(); iter != store.end(); iter++)
    {
        Serial.println("\nDeleting the following element: \n");
        Serial.print("Key: ");
        Serial.println(iter->first);
        delay(1000);
        Serial.print("Value: ");
        Serial.println(iter->second->ShowMeWhatYouGot());
        delay(1000);

        delete iter->second;
        iter->second = nullptr;
        store.erase(iter);
    } 

    if(store.empty())
       return true;
    else
       return false;
}

block where objects are allocated (delete_objects() is called right before this)

unsigned int index = 0;
unsigned int key = 0;
while(index < buff_string.length()){
    int amount = (rand() % 51) + 5;
    DummyClass* dc = new DummyClass(buff_string.substring(index, index + amount));
    store[key] = dc;
    index += amount;
    key++;
    Serial.println("Created new DummyClass object");
    Serial.print("Free Memory: ");
    Serial.println(System.freeMemory());
}

class definition

using namespace std;

class DummyClass {
    private:
        char* _container;

    public:
        DummyClass(){
        }

        DummyClass(String input){
            _container = new char[input.length()];
            strcpy(_container, input.c_str());
        }

        ~DummyClass(){
            delete _container;
            _container = nullptr;
        }

        char* ShowMeWhatYouGot(){
            return _container;
        }
};

Serial Output, it SOS’s when key is 0

Key: 194
Value: q->@B{s?tX/miN3K?[~2[mvTOSKPE34#5[@q8*d8BZwH`

Deleting the following element:

Key: 193
Value: 9Hb^7-J(#z3H#0uqg

Deleting the following element:

Key: 192
Value: V$xZ>C$u{MjxzxL?{KQqvQp*MN3~Ce&yZbmop1$9xlkJM)jGja~P{mY

Deleting the following element:

Key: 191
Value: yo*CVzE~2

Deleting the following element:

Key: 190
Value: [&PQakTV3{^Aq?(ffV/*24xaXej]~T1^SfWiM3ATpk#{CoQ

Deleting the following element:

Key: 0
Value: