How can I fix 'null' or 'undefined' errors when accessing object properties?

These errors, often 'TypeError: Cannot read properties of null (reading 'propertyName')', occur when you try to access a property on a variable that is `null` or `undefined`. Solutions include checking if the variable exists before accessing its properties (e.g., `if (myVariable) { ... }`), using the optional chaining operator (`?.`) (e.g., `myVariable?.propertyName`), or providing default values using the nullish coalescing operator (`??`) (e.g., `myVariable?.propertyName ?? defaultValue`).