SUMMARY
In some cases Umbrello wont create a UML diagram relation (when importing from code (python)) between two objects

STEPS TO REPRODUCE
1. Import the following python code via the code import wizard:
import abc
class Entity(abc.ABC):
    def __init__(self, entityNameStr):
        self.name = entityNameStr
        
class Consumer(Entity):
    def __init__(self, consumerNameStr):
        super(Consumer, self).__init__(consumerNameStr)
        self._consumersPropertySelectedLst = []

    def selectPropertyToView(self, propertyObj):
        cp = ConsumersPropertySelected(self, propertyObj)
        cp.addConsumersPropertySelected()
        
    def toString(self):
        print("Consumer Object:", self.name)
        print(self.name + " has selected the following property(ies) for viewing:")
        for cp in self._consumersPropertySelectedLst:
            print("-", cp._propertyObj.address)
        
class ConsumersPropertySelected(object):
    def __init__(self, consumerObj, propertyObj):
        self._consumerObj = consumerObj
        self._propertyObj = propertyObj

    def addConsumersPropertySelected(self):
        if self not in self._consumerObj._consumersPropertySelectedLst: #check if not already added
            self._consumerObj._consumersPropertySelectedLst.append(self)
            self._propertyObj._consumersPropertySelectedLst.append(self)

class Property(object):
    def __init__(self, addressStr):
        self._consumersPropertySelectedLst = []
        self.address = addressStr
        
    def toString(self):
        print("Property Object:", self.address)
        print(self.address + " is to be viewed by the following consumer(s):")
        for cp in self._consumersPropertySelectedLst:
            print("-", cp._consumerObj.name)
            
if __name__ == "__main__":
    pKlip1 = Property("Klip1")
    pGamka42 = Property("Gamka42")

    cPete = Consumer("Pete")
    cPete.selectPropertyToView(pGamka42)
    cPete.selectPropertyToView(pKlip1)

    cPete.toString()
    print()
    pGamka42.toString()
    print()
    pKlip1.toString()
2. Add the 'Property' & 'ConsumerPropertySelected' to the UML diagram
3. Select the aggregation relation from the toolbar
4. First left click on the 'Property' Object then left click on the 'ConsumerPropertySelected'

OBSERVED RESULT
See the image on the stack overflow page:
https://stackoverflow.com/questions/56739228/umbrello-wont-create-relation/56739597#56739597
Umbrello did not create the relation

EXPECTED RESULT
A relation is expected to be created between the two objects

SOFTWARE/OS VERSIONS
Umbrello: 4.14.65
Windows: Windows 10
macOS: N/A
Linux/KDE Plasma: N/A
(available in About System)
KDE Plasma Version: N/A
KDE Frameworks Version:N/A 
Qt Version: N/A (PyQt5-5.11.2)

ADDITIONAL INFORMATION
Workaround:
(refer to the stackoverflow link provided above)
1. Select the relation you want from the toolbar.
2. Left Click and hold on the first object(ex: 'Property')
3. Whilst still holding the left mouse button, drag the cursor to the second object (ex: 'ConsumerPropertySelected')
4. Release your left mouse button over the second object and finally click the second object again with your left mouse button